For extensive-stage small cell lung cancer (SCLC), platinum-chemotherapy (cisplatin or carboplatin) combined with etoposide as the first-line treatment is recommended. However, the recurrence rate is extremely high after the initial first-line treatment. For those who relapse or progress within more than six minths after the end of the-first-line treatment, chose the current guideline recommendation trearment plan has the poor prognosis. A new immunotherapeutic strategy is needed to achieve better anti-tumor effects. JS004 is a new antibody targeting B and T lymphocyte attenuator (BTLA), which restrains immune cells; function and leads to immune escape of tumor cells. The combination of PD-1 and BTLA antibodies has shown a good therapeutic effect in solid tumors. This trial aims to investigate the efficacy and safety of the therapeutic regimen of toripalimab and JS004 combined with platinum-based doublet chemotherapy in extensive-stage SCLC or relapsed within more than six months after radical treatment SCLC.

Inclusion Criteria:

1. The patient shall sign the Informed Consent Form;
2. Aged 18 ≥ years;
3. Histological or cytological diagnosis of SCLC by needle biopsy or EBUS, and extensive stage or recurrent confirmed by imageological examinations or relapsed within more than six months after radical treatment SCLC;
4. Eastern Cooperative Oncology Group (ECOG) performance-status score of 0 or 1;
5. Life expectancy is at least 12 weeks;
6. At least 1 measurable lesion according to RECIST 1.1;
7. Patients with good function of other main organs (liver, kidney, blood system, etc.);
8. Fertile female patients must voluntarily use effective contraceptives from the start of the trial to within 30 days after its completion, and urine or serum pregnancy test results within 7 days prior to enrollment are negative;
9. Unsterilized male patients must voluntarily use effective contraception during the trial period and for 30 days after its conclusion.

Exclusion Criteria:

1. Patients with a malignancy other than SCLC within five years prior to the start of this trial;
2. Participants with any unstable systemic disease (including uncontrolled hypertension, severe arrhythmia, etc.);
3. With activate or suspectable autoimmune disease, or autoimmune paracancer syndrome requiring systemic treatment;
4. Participants who are allergic to the test drug or any auxiliary materials;
5. Participants with active hepatitis B, hepatitis C or HIV;
6. Participants with Interstitial lung disease currently;
7. Pregnant or lactating women;
8. Any malabsorption;
9. Participants suffering from nervous system diseases or mental diseases that cannot cooperate;
10. Other factors that researchers think it is not suitable for enrollment.

The researchers will consider whether IPD is available to other researchers only after the paper is published.