Inclusion Criteria:
* Able to provide written informed consent form (ICF) and agree to follow study requirements and assessment schedule.
* Aged 18 years or older.
* Histologically confirmed stage I non - small cell lung cancer (AJCC 9th edition), with tumor size 2cm \<= T \<=4cm.
* Postoperative pathological report shows at least one high-risk factor (visceral pleural invasion, lymphovascular invasion, STAS, poorly differentiated status, high-grade invasive adenocarcinoma (any structure + high grade structure \>=20%, including solid, micropapillary, or complex glands)).
* ECOG performance status 0 or 1.
* PD-L1 expression \>=1%.
* No EGFR/ALK sensitive mutations.
* Achieved complete resection (R0) .
* Within 8 weeks after surgery, with full recovery from operation.
* Adequate organ function.
Exclusion Criteria:
* Any previous treatment for current lung cancer, including radiotherapy and systemic anti-tumour therapies (chemotherapy, immunotherapy, targeted therapy, anti-angiogenesis therapy, etc.).
* Prior chest radiotherapy (including lung, oesophageal, mediastinal, or breast cancer).
* Patients with large - cell neuroendocrine carcinoma (LCNEC) or mixed - subtype non - small - cell lung cancer with small - cell components.
* With EGFR/ALK sensitive mutations.
* Underwent segmentectomy or wedge resection only.
* Tumours involving main bronchi, or with obstructive pneumonia/atelectasis (partial or whole lung).
* Active autoimmune disease or history of relapsing autoimmune disease.
* History of interstitial lung disease, drug-induced interstitial lung disease, or radiation pneumonitis needing hormone therapy, or current active interstitial lung disease, or on relevant treatment/intervention.
* Any condition needing systemic corticosteroid (\> 10 mg/d prednisone or equivalent) or other immunosuppressant within 14 days before randomisation
* Used other approved systemic immunomodulators (interferon, interleukin - 2, tumour necrosis factor, thymopentin, thymosin α1, etc.) within 4 weeks before first dose.
* Herbs used for cancer control within 14 days before first study
* Live/attenuated vaccine receipt within 4 weeks before enrollment, or plan to receive during study or within 5 months after last tislelizumab dose.
* History of significant disease or conditions affecting organ/system function, per investigator's judgment.
* Severe chronic/active infection needing systemic antibacterial, antifungal, or antiviral therapy (e.g., tuberculosis) within 14 days before first study-drug dose. ·Known HIV infection.
* Allogeneic stem - cell/organ transplant history.
* Active malignancy within 2 years before enrollment, except the specific cancer studied and locally recurrent cancers cured (e.g., excised basal/squamous - cell skin cancer, superficial bladder cancer, cervical/ breast carcinoma in situ).
* Specific conditions and/or alcohol/drug abuse or dependence that may hinder drug administration, affect outcome interpretation, or increase complication risks.
* Pregnant/breastfeeding women, or men/women planning to conceive during the study.
* Participation in another interventional clinical study (except observational studies or follow-up phases).
