To evaluate the efficacy and safety of chemoradiotherapy combined with tirelizumab in the treatment of initial unresectable locally advanced gastric cancer or gastroesophageal junction cancer.

Inclusion Criteria:

1. Male or female patients, aged ≥ 18 years; ≤ 75 years old;
2. The histopathology confirmed the diagnosis of previously untreated locally advanced unresectable HER2 negative gastric or gastroesophageal junction adenocarcinoma (imaging evaluation showed that the primary tumor was severely invaded, could not be separated from the surrounding normal tissue, or had surrounded large blood vessels, or regional lymph nodes were fixed and fused, or the metastatic lymph nodes were not within the scope of surgical clearance), and there was no peritoneal metastasis during laparoscopic exploration;
3. There is no serious liver and kidney function damage, and the functional level of organs must meet the following requirements: ANC ≥ 1.5 × 109/L； PLT ≥ 90 × 109/L； Hb ≥ 90 g/L； TBIL ≤ 1 × ULN； ALT and AST ≤ 1.5 × ULN，ALP ≤ 2.5 × ULN； Bun and Cr ≤ 1 × ULN and creatinine clearance ≥ 50 ml / min (Cockcroft Gault formula); LVEF ≥ 50%； The QT interval (QTCF) corrected by fridericia method was < 450 ms in males and < 470 MS in females; INR ≤ 1.5 × ULN，APTT ≤ 1.5 × ULN；
4. The patient has at least one measurable lesion, which is evaluated by the investigator according to (RECIST) v1.1;
5. ECoG PS score 0 or 1;
6. Life expectancy ≥ 6 months;
7. The investigator assessed that the patient could comply with the protocol requirements;
8. Sign the informed consent document.

Exclusion Criteria:

1. Received systemic cytotoxic drug chemotherapy;
2. Patients with hypersensitivity or hypersensitivity to therapeutic drugs and patients with autoimmune diseases; Have received allogeneic tissue / solid organ transplantation;
3. There is third space effusion (such as pleural fluid and ascites) that cannot be controlled by drainage or other methods;
4. Use steroids for more than 50 days, or need to use steroids for a long time;
5. Uncontrolled symptomatic brain metastases or mental disorders cannot correctly express subjective symptoms;
6. The abnormal coagulation function has clinical significance, has bleeding tendency or is receiving thrombolytic or anticoagulant treatment;
7. Unable to swallow, chronic diarrhea and intestinal obstruction, and there are many factors affecting drug taking and absorption;
8. Other malignant tumors occurred within 5 years before enrollment, except for cervical carcinoma in situ or skin squamous or basal cell carcinoma that had been treated for radical cure before;
9. Pregnant or lactating women who have fertility but refuse to take contraceptive measures;
10. Those with serious heart disease or medical history, including: recorded history of congestive heart failure, high-risk uncontrollable arrhythmia, angina pectoris requiring medical treatment, clinically clear heart valve disease, history of serious myocardial infarction and stubborn hypertension;
11. According to the judgment of the investigator, there are concomitant diseases (such as uncontrolled hypertension, diabetes, thyroid disease, etc.) that seriously endanger the patient's safety or affect the patient's completion of the study;
12. The investigator determines that other conditions are not suitable for inclusion in the study.