Background and Objectives: The impacts of nutritional status on clinical outcomes in children receiving umbilical cord blood stem cell transplantation (UCBT) are not fully described. We evaluated the risk for malnutrition before transplantation admission and influence of weight loss during hospitalization on short-term clinical outcomes in children with UCBT. Methods and Study Design: We conducted a retrospective study of pediatric patients up to age 18 years who received UCBT and were treated at the Children’s Hospital of Fudan University between January 2019 and December 2020. Results: The mean age of the 91 patients was 1.3 years, with 78 (85.7%) men and 13 (14.3%) women (p<0.001). UCBT was performed mostly for primary immunodeficiency disease (PID) (83, 91.2%). The weight loss differences among children with different primary diseases were statistically significant (p=0.003). Children with a large amount of weight loss during hospitalization (n = 24) had higher risks of skin graft-versus-host disease (GVHD) (multivariate OR=5.01, 95% CI: 1.35-18.65), intestinal GVHD (multivariate OR=7.27, 95% CI: 1.74-30.45), a longer median hospital stay (p=0.004), higher antibiotic costs (p=0.008) and higher total hospitalization costs (p=0.004). Malnutrition on admission was significantly positively correlated with longer parenteral nutrition (PN) time (p=0.008). Early nutritional intervention effects on clinical outcomes need further assessment. Conclusions: Underweight recipient child and excessive weight loss during transplantation increases the length and cost of hospital stay, and is associated with a high incidence of GVHD, which affects the prognosis of transplantation and medical resources consumption.