This study is a phase Ib/III clinical study to evaluate the efficacy and safety of recombinant fully human anti-programmed cell death receptor 1 (PD-1) and anti-programmed cell death ligand 1 (PD-L1) bispecific antibody injection (IBI318) in combination with paclitaxel versus placebo in combination with paclitaxel in subjects with small cell lung cancer who have failed first-line or above chemotherapies.

Inclusion Criteria:

1. Signed the Informed Consent Form;
2. Male or female ≥ 18 and≤75 years of age;
3. Life expectancy ≥ 12 weeks;
4. Pathologically confirmed limited or extensive stage small cell lung cancer according to Veterans Lung Administration Lung Study Group (VALG) staging;
5. At least had progressive disease during or after platinum-based first-line chemotherapy or chemoradiotherapy, and with clear evidence of radiographic progression.

Exclusion Criteria:

1. Previous exposure to immune-mediated therapy; previous use of taxane chemotherapy;
2. Received the last anti-tumor therapy (chemotherapy, radiotherapy) within 4 weeks prior to the first dose of study drug;
3. Received any investigational agent within 4 weeks prior to the first dose of study drug;
4. Received systemic treatment with Chinese herbal medicine indicated for cancer or drugs used for immunoregulation (including thymosin, interferon, interleukin, except for local use for pleural effusion) within 2 weeks before the first dose;
5. Are participating in another interventional clinical study, or observational (non-interventional) clinical study or in the follow-up phase of an interventional study;
6. Pregnant or breastfeeding woman.