The windmill condition of turboprop engine before air-start will produce a certain amount of windmilling drag, which will seriously affect the operability and stability of the aircraft, so it is necessary to accurately calculate the windmilling drag characteristics to solve the special risk. Aiming at the characteristics of windmill in the flight of a submerged engine, a calculation and evaluation scheme was established based on the correction principle of the characteristic diagram of the standard propeller. Based on the flight-test of a certain turboprop engine, the flight test verification under different conditions was carried out. The results show that: at the same height, in the case of low speed windmill, the windmilling drag increases with the increase of speed, while the windmilling drag decreases with the increase of speed in the case of high speed windmill. With the increase of height, the windmilling drag of both low and high speed decreases. In the case of low speed windmill, the blade angle is basically in the limit position, while the rotation speed increases with the speed. In the case of high speed windmill, the rotation speed reaches the equilibrium speed, the blade angle increases with the speed. It is concluded that the windmilling drag calculation method of turboprop engine established in this paper is reasonable and feasible, and the accuracy of the calculation result meets the requirements of flight test, which can provide technical support for the flight test of the following turboprop engine in the air-start subject.