Objective To investigate the efficacy, prognostic factors and adverse reactions of intravesical instillation with Bacillus Calmette-Guérin (BCG) in patients with non-muscle-invasive bladder cancer (NMIBC). Methods The clinical data of 147 patients with moderate-to-high risk NMIBC who received BCG intravesical instillation after transurethral resection of bladder tumor (TURBT) in the Department of Urology of our hospital from Apr. 2014 to Apr. 2021 were retrospectively analyzed. Cumulative recurrence rate, cumulative progression rate, 1-year relapsefree survival rate, mean relapse-free time and mean progression-free survival time were used to evaluate the efficacy of BCG intravesical instillation, univariate and multivariate Cox regression analyses were used to explore the relapserelated risk factors, and Common Terminology Criteria for Adverse Events (CTCAE) Vesion 5.0 was used to evaluate adverse reactions. Results There were a total of 147 patients including 128 males (87.1%) and 19 females (12.9%), with a median follow-up time of 25 (4-85) months and an age of 39-98 (66.5±10.6) years. The 1-year relapsefree survival rate was 81.9% (104/127), the cumulative recurrence rate was 21.8% (32/147), and the mean relapsefree survival time was 49.49 months; the cumulative progression rate was 9.5% (14/147), and the mean progressionfree survival time was 56.81 months. Univariate analysis showed that relapse history (P=0.004) and pathological stage (P=0.069) of bladder cancer were correlated with tumor relapse. Multivariate Cox regression analysis showed that the relapse history of bladder cancer (hazard ratio[HR]=2.081, 95% confidence interval[CI]1.048-4.130, P=0.036) was an independent predictor of cancer relapse after BCG intravesical instillation. Adverse reactions occurred in 86.6% (110/127) patients during BCG intravesical instillation, including grade 1 adverse reactions in 38 cases (29.9%), grade 2 adverse reactions in 69 cases (54.3%), and grade 3 adverse reactions in 3 cases (2.4%), and there were no grade 4 or 5 adverse reactions. There were 9.5% (14/147) patients who discontinued treatment due to intolerance of adverse reactions. Conclusion Intravesical instillation with BCG after TURBT is effective in patients with moderate-to-high risk of NMIBC, and relapse history of bladder cancer is an independent predictor for tumor relapse. Some adverse reactions may occur in patients during BCG intravesical instillation, but most of them can be tolerated.