Inclusion Criteria:
* 1\. Age ≥ 40 years, regardless of gender;
* 2\. The investigator confirms the clinical diagnosis of IPF in participants based on chest HRCT, surgical lung biopsy, or transbronchial lung cryobiopsy (if available) performed during the screening period or within 1 year prior to screening (see Appendix 13.7 for details);
* 3\. FVCpp ≥ 45% during the screening period;
* 4\. Hemoglobin-corrected DLCOpp ≥ 25% and \< 90% during the screening period;
* 5\. Received a single stable-dose antifibrotic therapy for at least 12 weeks prior to screening (concurrent use of nintedanib and pirfenidone is prohibited) and will continue after randomization; or had not received stable antifibrotic therapy, or had discontinued such therapy for at least 8 weeks, with no plan to initiate antifibrotic therapy during the trial;
* 6\. Understands the purpose and risks of this study, comprehends and agrees to comply with all study procedures, consents to participate, and provides written informed consent.
Exclusion Criteria:
* 1\. Interstitial lung disease other than IPF.
* 2\. Airway obstruction during screening (FEV₁/FVC \< 0.7), or emphysema greater than pulmonary fibrosis on HRCT.
* 3\. Confirmed or suspected acute exacerbation of IPF within 3 months prior to screening.
* 4\. Investigator judgment that IPF severity showed sustained improvement during the 12 months prior to screening, based on changes in FVC, DLCO and/or HRCT findings.
* 5\. Other clinically significant respiratory diseases during screening.
* 6\. Severe diseases in any other system (cardiovascular, digestive, neurological, hematological, endocrine) during screening.
* 7\. Malignancy within 5 years prior to screening (excluding treated basal cell carcinoma of the skin, in situ squamous cell carcinoma of the skin, or carcinoma in situ of the cervix).
* 8\. Any acute infection within 2 weeks prior to screening that has not fully recovered per investigator judgment.
* 9\. Active, unstable or uncontrolled vasculitis within 8 weeks prior to screening.
* 10\. Any acute or chronic active infection during screening.
* 11\. C-SSRS assessment during screening indicating suicidal behavior within the past 2 years (actual attempt, interrupted attempt, aborted attempt, or preparatory acts or gestures), or clinically significant suicidal ideation within 3 months prior to screening or during screening (participant answered "yes" to C-SSRS suicidal ideation question 4 or 5).
* 12\. Treatment with PDE1, PDE3, PDE4, PDE10 inhibitors, or non-selective PDE inhibitors within 4 weeks prior to screening.
* 13\. Use of strong CYP3A4 inhibitors or inducers within 14 days or 5 half-lives (whichever is longer) before the first dose of investigational product, or inability to discontinue such agents during the study.
* 14\. Receiving immunomodulatory agents (excluding oral glucocorticoids) for respiratory or pulmonary conditions during screening, or prednisone (or equivalent) at a daily dose \> 15 mg.
* 15\. Abnormal hepatic and renal function during screening: ALT, AST \> 2.5 × ULN, or TBIL \> 1.5 × ULN, or eGFR \< 30 mL/min/1.73 m².
* 16\. Severe, persistent, uncontrolled hypertension during screening (SBP ≥ 180 mmHg or DBP ≥ 100 mmHg).
* 17\. History of smoking within 3 months prior to screening or unwillingness to abstain from smoking (including e-cigarettes) during the study.
* 18\. Hypersensitivity to SYH2059 or any excipients, or history of severe drug allergy.
* 19\. Participation in any clinical trial within 4 weeks prior to screening (excluding those not receiving investigational product).
* 20\. Participation in a clinical study of the same target drug and receipt of treatment within 3 months prior to screening.
* 21\. Pregnant or lactating females; fertile females or males unwilling to practice strict contraception throughout the trial and for 3 months after trial completion until the end of the safety follow-up period (including male participants).
Any other conditions deemed inappropriate for trial participation by the investigator.
* 22\. Additional Exclusion Criteria (for PK intensive sampling participants):
* 23\. Previous history of gastrointestinal surgery that may interfere with the PK of the investigational product.
* 24\. Alcohol consumption exceeding 14 units per week within 4 weeks prior to screening.
* 25\. Habitual excessive intake of xanthine- or caffeine-containing foods, beverages, or other substances affecting drug absorption, distribution, metabolism or excretion within 4 weeks prior to screening.
