Many rivers in the region of northwest China are drying up, and the ecological environment is getting worse. Studying methods of calculating the ecological water requirement (EWR) for dried-up rivers will help to slow down the deterioration of the ecological environment and conserve biodiversity. The water requirement of vegetation and infiltration (WRVI) method is proposed in this paper. This method focuses on dried-up rivers and takes the water requirement of vegetation and river bed infiltration into consideration. This is different from the conventional methods, which only focus on the rivers that have a flow rate. Due to drying, the ecological environment is worsening year by year in the lower reaches of the Zhang River in the Haihe River Basin in northwest China. This river is used as an example to determine the EWR, and the results are compared with another method. The results show that the WRVI method can calculate the EWR more accurately by considering various factors in different years and months for dried-up rivers.