Safety and Efficacy of BCG Combined With Tislelizumab for BCG-untreated Patients With High-risk Non-muscle Invasive Bladder Cancer

Inclusion Criteria: * ≥ 18 and ≤75 years old on day of signing informed consent * Signing informed consent * Patients with histologically confirmed high-risk NMIBC after TURBT (Patients with mixed histology, predominantly transitional cells, could be enrolled.) High grade pathology (any of the following conditions) * CIS * T1 * \>3cm * Multifocal * Patients must be willing to provide a blood sample and a TURBT specimen must be taken at baseline. * For patients with T1 or suspected incomplete tumor resection after first TURBT, incomplete initial resection or no muscle in original specimen, they should undergo TURBT again within 2-6 weeks. * No distant metastasis confirmed by CT or MRI in the chest, abdomen, or pelvic cavity within 42 days before treatment. * ECOG performance status of ≤2 * Life expectancy ≥12 weeks * Well-controlled blood pressure and within 7 days before treatment \<160/95mmHg * Normal organ function within 7 days before treatment * HB≥90 g/L * ANC≥1.5×109 /L * PLT≥100×109 /L * T-BIL≤1.5×ULN * ALT, AST≤2.5×ULN * eGFR≥ 20ml/min * INR, APTT≤1.5× ULN. Exclusion Criteria: * Received prior therapies targeting PD-1 or PD-L1. * Received prior intravesical therapy of BCG. * Receive any approved anticancer therapy, including systemic and intravesical chemotherapy within 21 days before enrollment. * Receive any other trial drug or participate in another therapeutic clinical study within 28 days before enrollment. * History of severe hypersensitivity reactions to other monoclonal antibodies. * History of other malignancy. * Active tuberculosis. * Severe infections occur within 4 weeks prior to enrollment, including but not limited to infectious complications leading to hospitalization, bacteremia, or severe pneumonia. * A known history of HIV infection. * Untreated chronic hepatitis B patients or hepatitis B virus carriers whose HBV DNA≥500 IU/mL * Patients with active hepatitis C * Participants with active autoimmune diseases or history of autoimmune diseases that may relapse * Clinically significant cardiovascular disease, including heart disease (NYHA ≥Ⅲ), myocardial infarction, unstable arrhythmia or unstable angina within 3 months before enrollment. * A known history of LVEF\<40% * Prior allogeneic stem cell transplantation or organ transplantation * History of interstitial lung disease, noninfectious pneumonitis, or uncontrolled diseases. * Underlying diseases that the investigator believes are not conducive to study treatment or difficult to explain by drug toxicity or adverse events. * Receive hormone therapy or other immunosuppressive therapy within 14 days before enrollment.