In order to improve the performance of an air turbine starter and match it with the bleed characteristics of the auxiliary power unit， the aerodynamic performance and flow field details of the air turbine starter are studied by means of numerical simulation and experiment， and the optimal design of the guide and turbine stage blade is completed. After the modification， the mass flow， the efficiency and the power increase by 23.2%， 3.4% and 18.0%， respectively. The comparison between numerical simulation and test results shows that the overall difference of flow is small， the fluctuation range is within ±3%， and the mean value is equal to 0.17%. The maximum power difference is equal to 2.38% and the minimum is equal to 0.1%. The numerical simulation results show that there are common working points between the modified air turbine starter and the bleed characteristic line of the auxiliary power unit， and the working performance of the auxiliary power unit has been significantly improved.