Background: Postoperative non-small cell lung cancer (NSCLC) patients frequently encounter a deteriorated quality of life (QOL), disturbed immune response, and disordered homeostasis. Si-Jun-Zi Decoction (SJZD), a well-known traditional Chinese herbal formula, is frequently employed in clinical application for many years. Exploration is underway to investigate the potential therapeutic effect of SJZD for treating postoperative NSCLC. Objective: To assess the efficacy of SJZD on QOLs, hematological parameters, and regulations of gut microbiota in postoperative NSCLC patients. Methods: A prospective observational cohort study was conducted, enrolling 65 postoperative NSCLC patients between May 10, 2020 and March 15, 2021 in Yueyang Hospital, with 33 patients in SJZD group and 32 patients in control (CON) group. The SJZD group comprised of patients who received standard treatments and the SJZD decoction, while the CON group consisted of those only underwent standard treatments. The treatment period was 4 weeks. The primary outcome was QOL. The secondary outcomes involved serum immune cell and inflammation factor levels, safety, and alterations in gut microbiota. Results: SJZD group showed significant enhancements in cognitive functioning ( P = .048) at week 1 and physical functioning ( P = .019) at week 4. Lung cancer-specific symptoms included dyspnea ( P = .001), coughing ( P = .008), hemoptysis ( P = .034), peripheral neuropathy ( P = .019), and pain (arm or shoulder, P = .020, other parts, P = .019) eased significantly in the fourth week. Anemia indicators such as red blood cell count ( P = .003 at week 1, P = .029 at week 4) and hemoglobin ( P = .016 at week 1, P = .048 at week 4) were significantly elevated by SJZD. SJZD upregulated blood cell cluster differentiation (CD)3+ ( P = .001 at week 1, P < .001 at week 4), CD3+CD4+ ( P = .012 at week 1), CD3+CD8+ ( P = .027 at week 1), CD19+ ( P = .003 at week 4), increased anti-inflammatory interleukin (IL)-10 ( P = .004 at week 1, P = .003 at week 4), and decreased pro-inflammatory IL-8 ( P = .004 at week 1, p = .005 at week 4). Analysis of gut microbiota indicated that SJZD had a significant impact on increasing microbial abundance and diversity, enriching probiotic microbes, and regulating microbial biological functions. Conclusions: SJZD appears to be an effective and safe treatment for postoperative NSCLC patients. As a preliminary observational study, this study provides a foundation for further research.