Objective To analyze the factors influencing the increase of extracellular water/total body water (ECW/TBW) in tumor patients lacking edema signs, and to provide reference for the diagnosis and timely management of early water retention in tumor patients. Methods 274 tumor patients admitted to Anqing First People's Hospital affiliated to Anhui Medical University during 2020.12-2022.12 were selected, excluding clinically observable edema. General clinical data, main laboratory test results and body composition indexes were collected,and the valuable variables were screened for linear regression analysis with ECW/TBW based on professional knowledge and univariate analysis results. Results Age, gender, hemoglobin, serum sodium,serum potassium and phase angle (PA) were independent influencing factors for elevated ECW/TBW. ECW/TBW increased with increasing age (partial regression coefficient B=0.009, P=0.001), ECW/TBW was higher in males than in females (B=-0.349, P<0.001), and hemoglobin (Hgb) negatively affected ECW/TBW (B=-0.003, P=0.039). Blood sodium positively affected ECW/TBW (B=0.019, P=0.011), while blood potassium had a negative effect (B=-0.180, P=0.001). PA was the factor with the greatest influence on ECW/TBW of all independent variables (B=-1.006, P<0.001). However, Performance Status (PS) score, Patient-Generated Subjective Global Assessment(PG-SGA) score, tumor stage, prealbumin, albumin, and body fat percentage were not independent influences for elevated ECW/TBW. Conclusion The early influencing factors of water retention in tumor patients are mainly due to the abnormal function of cell membrane, but also affected by age, gender, anemia and electrolyte, and timely body composition testing can help in the next intervention decision making.