Abstract: By using FLUENT software, the numerical simulation of counter-rotating fan with external motor was performed, variation conditions of inner flow field were analyzed. Flow field of the counter-rotating fan was simulated and analyzed in different kinds of hub-tip ratio, get the influence about hub–tip ratio to the aerodynamic parameters of counter-rotating fan with external motor from detail comparisons and analyses, such as total pressure, mass-flow, efficiency. It could provide a useful reference to the optimal design of counter-rotating fan.