A numerical simulation of a mixed-flow compressor under windmill conditions is conducted to investigate the cause of stagnation pressure loss captured in the previous experiment, and to clarify the qualitative flow structure under windmill conditions. As a result of the simulation, the compressor rotor causes a stronger load on the airflow even under windmill conditions, because the compressor is connected to the turbine. It turns out that the mass flow rate increases in the order of rated operating point: windmill conditions with turbine and windmill conditions without turbine. In addition, a negative angle of attack is confirmed for the first stator under windmill conditions, which leads to a large separation on the pressure-side of the blade. This tendency increases when the rotational speed decreases. By a simple analysis, it is clarified that the lower the pressure ratio and rotational speed, such as under windmill conditions, the further the rotor and stator angle of incidence become separated from the optimal degree.