Objective To investigate the relationship between postoperative body mass index(BMI) and prognosis in patients with primary gastric cancer(PGC). Methods 362 cases of PGC in Bazhong Central Hospital from January 2011 to December 2016 were selected as the research objects. According to the Chinese standard of BMI, they were divided into low weight (BMI<18.5 kg/m2) , normal weight(BMI:18.5-23.9 kg/m2 ), over weight(BMI:24.0-27.9 kg/m2) and obesity(BMI≥28.0 kg/m2 ). Kaplan⁃Meier method was used to calculate OS and DFS in 4 groups , Log rank test was used to compare the differences among the 4 groups. Cox proportional hazard model was used to evaluate the prognosis factor analysis. Results The 3⁃year DFS of PGC with low weight, normal weight, overweight and obesity were 90.0%, 77.4%, 78.9% and 79.8%, respectively. By Log⁃rank test, there was significant difference between the groups (χ2= 1.110, P=0.832); the 5⁃year OS of low weight, normal weight, overweight and obese PGC were 32.3%, 86.9%, 71.5% and 69.0%, respectively, and the difference was statistically significant (χ2=7.616, P=0.027), and further pairwise comparison found that the OS level of low weight and obesity patients was the lowest. Multivariate analysis showed that obesity (BMI≥28.0 kg/m2) patients (RR=1.559, 95%CI= 1.262-2.420, P=0.022) and underweight (BMI≤18.5 kg/m2) patients (RR=3.385, 95%CI=1.698-6.767, P=0.002) were independent risk factors for postoperative survival of patients with PGC. Conclusion Postoperative underweight and obesity can be considered as independent factors affecting the survival rate of primary gastric cancer.