Background: This study conducted a large-scale multi-center analysis using real-world data from China to comprehensively investigate the clinical characteristics and prognosis of patients with deficient mismatch repair (dMMR) or high microsatellite instability (MSI-H) gastric and colorectal cancer undergoing curative surgery. Additionally, we analyzed postoperative adjuvant treatment effects and recurrence/metastasis patterns. Methods: We retrospectively analyzed 1800 gastric and colorectal cancer patients with dMMR and/or MSI-H confirmed by immunohistochemistry (IHC) and PCR after radical resection from 19 hospitals in China. We compared clinical characteristics among positive patients identified by both testing methods and evaluated their relationship with disease-free survival (DFS) and overall survival (OS). Results: The cohort included 1702 dMMR patients (regardless of MSI status), 667 MSI-H patients (regardless of MMR status), and 46 cases with inconsistent MMR and MSI statuses. No significant differences in overall clinical-pathological characteristics were observed between IHC-dMMR and PCR-MSI-H patients, highlighting the clinical utility of both approaches. Predominantly, MMR protein defects involved MLH1/PMS2 loss (>70%). We found a positive correlation between lymph node metastasis and MLH1/PMS2 protein deficiency (P<0.01) and an inverse correlation trend in cases of MSH2/MSH6 deficiency (P<0.1). Additionally, poorer prognosis was linked to alcohol consumption (DFS, P<0.001), lymph node metastasis (DFS, P<0.001; OS, P<0.001), vascular cancer emboli (OS, P<0.001), and neural invasion (OS, P<0.001). In the dMMR/MSI-H colorectal subgroup, colon cancer patients exhibited superior DFS compared to rectal cancer (P=0.025) and improved DFS with right colon tumors (P=0.01). Patients with MLH2/MSH6 deficiency showed a trend towards better survival, even within the subgroup receiving adjuvant chemotherapy (P=0.067), compared to MLH1/PMS2 and other deficiency disorders. Notably, patients with dMMR and/or MSI-H exhibited a shorter DFS following adjuvant therapy (P<0.001), while OS did not achieve a significant difference, potentially due to the insufficient follow-up duration. Conclusions: This study provides valuable insights into the correlation between MSI-H/dMMR status and clinical characteristics in Chinese patients with gastric and colorectal cancer after radical resection. The findings also illuminate prognostic factors impacting DFS and OS, as well as the intricate relationship between MMR/MSI status and tumor metastasis patterns. It highlights the feasibility of utilizing both MMR-IHC and MSI-PCR for patient selection and enhances our understanding of the clinical management and outcomes of patients with dMMR/MSI-H gastric and colorectal cancer.