Gastric cancer is a major cause of cancer death worldwide, especially in China, and the nutritional status of patients fundamentally affects the prognosis. A cross-sectional study was conducted between January and May 2024 in the Department of Medical Oncology, Inner Mongolia Hospital, Peking University Cancer Hospital and included 406 family members of patients. Median knowledge scores were 27.00 (25th and 75th percentiles: 19.00; 33.00), median attitudes 31.00 (30.00; 36.00), and median practices 36.00 (34.00; 42.00). Out of 406 respondents, 216 (53.2%) were women. Structural equation modeling showed that income, nursing support, and training significantly influenced family members' knowledge, attitudes, and practices (all p < 0.05). Mediation analyzes also demonstrated indirect effects of these factors on attitudes and practices (all p < 0.05). The authors conclude that family members have a relatively good understanding and positive attitudes towards nutritional care, but their actual practices are insufficient, and educational interventions are needed to improve them.