Objectives　To explore the relationship of hyponatremia with severity and prognosis of heart failure in children.Methods　The serum sodium level, clinical characteristics and prognosis of heart failure in 171 children were analyzed retrospectively. Results　In the 171 children (90 males, 81 females), an average age was 3.0±1.1 years. There were 72 cases of mild heart failure, 54 cases of moderate heart failure, and 45 cases of severe heart failure. Among 171 children, 103 (60.23%) had hyponatremia. The incidence of hyponatremia in children with mild, moderate and severe heart failure was 48.61%, 55.56% and 84.44% respectively The differences were statistically significant (P<0.001). The differences in serum sodium levels among the three groups were also statistically significant (P<0.001), and serum sodium levels gradually decreased with the aggravation of heart failure. The mortality of children with mild, moderate and severe heart failure complicated with hyponatremia was 2.86%, 20.00% and 71.05% respectively. The differences were statistically significant (P<0.001). There was a significant negative correlation between serum sodium level and the severity of heart failure (r=0.28, P<0.001). Logistic regression analysis showed that the risk of death was higher at lower level of serum sodium, (OR=0.16, 95%CI: 0.06~0.44). Conclusions　Hyponatremia is closely related to the severity and prognosis of heart failure in children.