ObjectiveTo investigate the long‐term changes in nutritional status in children with solid tumors during treatment and the relationship between nutritional status and clinical outcomes.MethodsThis study was a retrospective medical records review of data from children who were diagnosed with solid tumors and followed up for more than 3 months from January 2016 to December 2021 in China. Patient demographics and clinical information, including nutritional status, parenteral nutrition use, intensive care unit (ICU) transfers, infection during hospitalization, hospitalization frequency, length of stay, hospitalization costs and antibiotic costs, were collected to analyze the nutritional status of children with different types of solid tumors, the dynamic changes in nutritional status during treatment, and the relationship between nutritional status and clinical outcomes.ResultsAmong the 764 patients (383 males (50.1%); 381 females (49.9%); mean age: 2.58 years), 41.6% of the solid tumors were neuroblastomas, 17.1% were hepatoblastomas, and Wilms tumors as the third most common solid tumors (8.9%). The median follow‐up duration was 6 months (range: 3–40 months). At diagnosis, the proportion of children who were undernourished (underweight and wasting) versus overweight or obese were 26.71% versus 5.21% (25.86% vs. 2.89% in the third month; 29.77% vs. 2.28% in the sixth month; 24.77% vs. 3.27% in the 12th month). The body mass index Z scores decreased from the initial values after the first month (−0.56 (−1.47, 0.23) vs. −0.44 (−1.29, 0.41)) but improved later and decreased again at 6 months. The children in the undernutrition group had longer hospital stays (p < 0.001), higher hospitalization costs (p < 0.001), higher antibiotic costs (p < 0.001), a higher risk of neutropenia (OR = 4.781 (95% CI: 1.571–14.553), p = 0.006), and a higher risk of ICU transfers (OR = 1.498 (95% CI: 1.010–2.224), p = 0.044). No significant differences in those associations by malnutrition and infection, ICU duration, or length of parenteral nutrition were observed.ConclusionThere is a considerable prevalence of malnutrition in children with solid tumors. Malnutrition is related to adverse clinical outcomes and increases in total hospital expenses and antibiotic costs.