Objective: To investigate the treatment and prognosis of multiple primary malignant neoplasms (MPMN) complicated with renal cell carcinoma (RCC), and to make risk stratification. Methods: A retrospective study of 27 cases of MPMN with RCC in two centers, including the different tumors of MPMN, specific treatment methods, and the interval between primary cancers. At the same time, the survival conditions, including recurrence, metastasis and survival, were followed up for statistical analysis. The interval between the two kinds of primary cancer within 6 months was simultaneous MPMNs, and more than 6 months was metachronous MPMNs. For simple risk stratification of cases, as long as one of the MPMNs had a stage Ⅲ or higher malignancy, which was defined as high risk. Results: Among the 27 patients, 20 were male and 7 were female, with age at the time of diagnosis was 42-82 years, with an average age of (61.3±11.7) years. The age at the diagnosis of renal cancer was 43-87 years, with an average age of (66.0±11.3) years. There were 21 cases with duplex primary malignant neoplasms, 4 cases with triple primary malignant neoplasms, and 2 cases with quadruple primary malignant neoplasms. The interval between first cancer and second cancer was 0-360 months, with a median of 18 months. There were 17 cases of metachronous multiple primary malignant neoplasms and 10 cases of simultaneous multiple primary malignant neoplasms. The most common system of MPMN with comorbid RCC involved urologic system, digestive system and respiratory system. The most common locations of MPMN with comorbid RCC were bladder cancer, lung cancer and colon cancer. Follow-up time calcu- lated from the last cancer was 2-156 months, with a median of 32 months. And 14 cases survived and 13 cases died, with 11 cases being tumor related. Tumor stage was the risk factor of prognosis. Any kind of tumor stage in stage Ⅲ or above had a relatively poor prognosis. Conclusion: MPMN complicated with RCC is relatively rare. Standard treatment should be used for each cancer type during the treatment process. The prognosis mainly depends on the highest stage of each tumor. Simple risk stratification shows that the prognosis of the high-risk group is worse. This simple stratification method may be helpful to predict the prognosis.