Abstract: This paper combines the hydraulic model and the physical habitat simulation model(PHABSIM)to calculate the instream ecological flow by using the Yellow River carp as target species in Lanzhou to Shizuishan reach of the Yellow River mainstream and taking water depth and velocity into consideration.By analyzing the ecological,hydrological and cross-sections data before the flood periods from 1975-1985,the ecological instream flows at Lanzhou,An'ningdu,Xiaheyan,Qingtongxia and Shizuishan station calculated by PHABSIM before flood are 195 m³/s,220 m³/s,220 m³/s,220 m³/s,175 m³/s respectively.And the result is compared with the instream ecological flow calculated by the Tennant method.The results calculated by PHABSIM are within the scope of the those calculated by Tennant.