This is a multicenter, randomized, open-label phase III study to determine the efficacy and safety of intravesical SHR-1501 combined with Bacillus Calmette Guerin (BCG) versus investigator-selected chemotherapy in patients with BCG-unresponsive papillary-only non-muscle-invasive bladder cancer (NMIBC).

Inclusion Criteria:

1. Aged at least 18 years, gender is not limited;
2. Eastern Cooperative Oncology Group (ECOG) performance status of ≤ 2;
3. Histologic confirmation of high-risk non-muscle invasive bladder cancer (high-grade Ta and/or any T1, no carcinoma in situ); BCG-unresponsive;
4. Cystoscopy within 6 weeks before the randomization shows that the lesion has been completely removed; for T1 stage lesions, pathological results must show the presence of bladder muscle tissue;
5. Not suitable for or unwilling to undergo radical cystectomy;
6. Sufficient hematology and organ function.

Exclusion Criteria:

1. Previous histopathological diagnosis was muscle-invasive, locally advanced or metastatic urothelial carcinoma (T2 or T3 or T4 or N+ or M+);
2. Received surgery such as TURBT or radiotherapy for bladder lesions within 2 weeks before the randomization; Major surgery was performed within 28 days before randomization;
3. Those who have previously received the following treatments and have not experienced disease recurrence or progression before enrolment: a) Intravesical infusion of cytotoxic chemotherapy or other drugs (such as oncolytic viruses), etc; b) Immune checkpoint inhibitors; c) Other investigatory drugs for the treatment of NMIBC;
4. Those who have previously received immunostimulant therapy;
5. Upper urinary tract tumor detected by CTU or MRU during screening period, urethral prostate tumor detected by cystoscopy, or other concomitant malignant tumors within 5 years before the first administration;
6. Previous medical history or examination suggests active tuberculosis within 1 year prior to the randomization.