Abstract: Background　Gastric cancer is a common malignancy worldwide with poor prognosis, and there is an urgent need for novel molecular biomarkers and therapeutic targets. Checkpoint kinase 1(CHEK1), a key regulator of the cell cycle, has not been fully characterized in gastric cancer. Objective　To systematically characterize the expression pattern, prognostic value, and mechanistic role of CHEK1 in gastric cancer, with a focus on cell-cycle regulation and remodeling of the tumor immune microenvironment. Methods　Pan-cancer and paired STAD tumor – normal analyses using TCGA and GEO datasets were performed to evaluate CHEK1 expression and its associations with overall survival and immune cell infiltration. SsGSEA was applied to calculate infiltration scores of 24 immune cell types and to analyze the correlations between CHEK1 expression and the infiltration levels of different immune cell subsets. Co-expression network construction and functional enrichment analyses were conducted to identify CHEK1-related signaling pathways. In vitro, CHEK1 knockdown and overexpression models were established in AGS cells. Cell proliferative activity was assessed by CCK-8 assay. Apoptosis was measured by flow cytometry, and apoptosisrelated proteins alterations were examined by western blotting, thereby providing functional and mechanistic validation. Results　CHEK1 was overexpressed in stomach adenocarcinoma (STAD) and 11 other cancer types (P＜0.05). In STAD, CHEK1 expression was significantly higher in tumor tissues than in paired adjacent normal tissues, and high CHEK1 expression was associated with a more favorable prognosis (P＜0.05). Co-expression and functional enrichment analyses indicated that CHEK1-associated genes were enriched in cell cycle–related pathways and were significantly correlated with multiple DNA damage repair–related genes (P ＜0.05). Immune infiltration analysis showed that CHEK1 expression was positively correlated with the Th2 cell infiltration score (R =0.567, P＜0.001) and negatively correlated with the infiltration scores of CD8⁺ T cells (R=−0.214, P＜0.001) and dendritic cells (R =−0.192, P＜0.01). In vitro, CHEK1 knockdown inhibited AGS cell proliferation (CCK-8 OD reduced by 42%, P＜0.01), increased apoptosis (39.28% vs 7.86%, P＜0.001), and upregulated γH2AX (P＜0.05), whereas CHEK1 overexpression decreased cleaved PARP and caspase-3 levels (P＜0.001), thereby suppressing apoptosis. Conclusion　CHEK1 promotes gastric cancer progression through a dual mechanism involving cell-cycle dysregulation and remodeling of the immunosuppressive tumor microenvironment. High CHEK1 expression predicts better prognosis and supports CHEK1 as a promising prognostic biomarker and therapeutic target, particularly in the context of combination strategies with immune checkpoint inhibitors.