Objective To measure the free fat mass (FFM) of hospitalized patients with malignant tumors and to analyze its influence factors. Methods The study selected 107 hospitalized patients from the Department of Medical Oncology. Among them, 31 were uppergastrointestinal tumors; 41 lower gastrointestinal tumors, 14 lung cancers; and 21 other types of tumors. General information (gender, age, diagnosis, disease duration, clinical stage, number of chemotherapy, history of cancer surgeries, history of chronic diseases, etc.) and relevant nutritional indexes (albumin, pre-albumin, and body mass index) of patients were collected. The FFM and basal metabolic rate (BMR) were evaluated by the bioelectrical impedance (BIA) method. The dietary energy and protein intake were evaluated based on the 24-hour dietary recalls. Results Among 107 patients, total incidence of malnutrition was 36.4%. The incidence of malnutrition of patients with upper gastrointestinal tumors and longer duration of disease (>1 year) was high, while the incidence of malnutrition of patients with adequate protein intake (≥75% of the recommended intake per day) was low. Energy and protein intake was positively correlated with FFM. FFM and corrected BMR of male patients decreased significantly with the increase of the number of chemotherapy (P<0.05). Only the corrected BMR of female patients with non-gastraintestinal tumors was significantly higher than that of patients with gastraintestinal tumors (P<0.05). The differences of other indexes were not statistically significant (P>0.05). Conclusion The incidence of malnutrition of patients with tumors was high. Adequate protein intake can improve the nutritional status. FFM and corrected BMR of male patients with malignant tumors decrease with the increase of the number of chemotherapy.