In order to improve the performance of an air turbine starter， the aerodynamic performance and flow field details of the air turbine starter are studied by combining numerical simulation and experiment. The numerical simulation results show that there is obvious flow separation inside the inlet elbow， leading to poor uniformity of the downstream flow field. There is a certain boundary layer separation on the surface of the guide vane and rotor blade， and the flow field is relatively poor because the smooth flow is not fully considered in the exhaust frame. The comparison between the numerical simulation results and the experimental results shows that the air flow rate obtained by numerical simulation is about 1.90% lower than that of the experiment， and the turbine power is about 1.04% higher. Generally speaking， the numerical simulation is in good agreement with the experiment， and this numerical simulation method has a high accuracy to study the air turbine starter. The performance of the air turbine is significantly improved by the optimized design of the guide vane. The power of the air turbine is increased by 14.2% compared with the prototype.