The impact of Coanda effect on the flow resistance coefficient of flat-walled diffusers has been studied by numerical simulation, and the results show that when Reynolds number changes in the range of 300~3 000 and the diffusion angle θ of the cone tube varies from 5° to 40°, the fluid flow to the diffusion direction goes through three stages: the stable state, the wall-attachment state and the jet deflection state, when 10°<θ<35°, the Coanda effect can be obtained easily. When Reynolds number ranging from 300 to 1 200, with the increasing of diffusion angle, the flow resistance coefficient decreases rapidly during the stable state, increases slowly among the wall-attachment state and decreases gradually in time of the jet deflection state. While Re varies from 1 800 to 3 000, the flow resistance coefficient of the wall-attachment state reaches maximum at θ=30°. The flow resistance coefficient ratio in steady state and jet condition state is basically unchanged, but decreases quickly during the wall-attachment state with the increasing of diffusion angle.