Objective To determine the effect of body mass index (BMI) on the prognosis of the elderly cardiovascular disease (CVD) patients with prostate cancer, and construct and validate a predictive nomogram for 1-year readmission of the patients due to CVD. Methods Clinical data of 237 elderly CVD patients with freshly diagnosed prostate cancer in the First Hospital of Shanxi Medical University from May 2019 to May 2021 were collected and retrospectively analyzed. According to the BMI criteria recommended by the Working Group on Obesity in China (WGOC), they were divided into high BMI group (n=110) and low BMI group (n=127). Their baseline data, laboratory results and imaging data were compared between the two groups. The primary endpoint was the first rehospitalization due to CVD and the secondary endpoint was all-cause death within one year after discharge. SPSS statistics 26.0 and R Studio (version 4.1.2) were used for data analysis. Student′s t test, Mann-Whitney U test or Chi-square test was employed for intergroup comparison depending on data type. Kaplan-Meier survival curve was plotted to analyze their survival. Multivariate Cox regression analysis was performed to analyze the independent prognostic factors and a nomogram of prognostic prediction was constructed. Results A total of 118 patients were rehospitalized due to CVD, including 71 cases from the low BMI group(55.9%) and 47 cases from the high BMI group (42.7%) . Among the four cases of all-cause death, three were in the low BMI group (2.4%) and one was in the high BMI group (0.9%). Kaplan-Meier survival curves revealed that the low BMI patients had higher rate of rehospitalization due to CVD than the high BMI patients, especially coronary heart disease (P<0.05). But there was no significant difference in all-cause mortality between the two groups (P>0.05). Multivariate Cox regression analysis showed that smoking (HR=3.954,95%CI 2.359-6.628; P<0.05), high total cholesterol (HR=1.070,95%CI 1.035-1.107; P<0.05) and comorbidities of other systems (HR=1.611,95%CI 1.103-2.351; P<0.05) were independent risk factors for rehospitalization for CVD, and high BMI (HR=0.920,95%CI 0.862-0.981; P<0.05) and high high-density lipoprotein cholesterol (HDL-C, HR=0.460,95%CI 0.245-0.865; P<0.05) were independent protective factors. The internal validation C-index of the constructed nomogram was 0.731 (95%CI 0.686-0.776). The AUC value was 0.759 (95%CI 0.689-0.829), 0.767 (95%CI 0.699-0.834) and 0.804 (95%CI 0.696-0. 912) respectively, at three, six and twelve months, suggesting that the model has a moderate predictive value and good consistency in the calibration plots. Conclusion Smoking, total cholesterol, other systemic diseases, BMI and HDL-C are independent factors for the prognosis of elderly CVD patients with prostate cancer. Our constructed nomogram has good predictive accuracy, and is helpful to assess the prognosis of the patients.