Abstract: To analyze the treatment strategy for a 78-year-old female patient with mismatch repair deficient (dMMR) gastric cancer who achieved long-term survival. After third-line chemotherapy failed, gene testing showed ARID1A p.Gln748fs, c.2733-1G＞T variation, with PD-L1 TPS 30%, CPS 60%. The nivolumab was employed, and two weeks later, the best response was partial response (PR). During the fourth-line immunotherapy maintenance treatment, progression of left adrenal metastasis was observed. The expression of human epidermal growth factor receptor-2 (HER-2) was positive, and the antibody drug conjugate disitamab vedotin (RC48) was chosen for treatment. After 10 months of treatment with nivolumab combined with RC48, the best efficacy was assessed as stable disease (SD), with a progression free survival (PFS) of up to 12 months. Radiotherapy was employed, and immunotherapy was maintained, allowing the patient to achieve a PFS of 18 months again. During immunotherapy, a clinical pharmacist developed a personalized pharmaceutical care plan for this patient. At the last follow-up, this patient achieved 78 months of long-term survival.