Nowadays，the aircraft using a three － stage wound － field synchronous generator has received more and more attention． In this system，the constant speed drive ( CSD) adopted for AC power systems is used to eliminate the influence caused by the rotational speed of engine． Usually，the current transformers ( CT) of the generator and lines are used for ensuring the proper operation of electrical system，which can avoid severe damage due to a defect． However，the mal － operation of differential protection will seriously interfere the security of aircraft． The structure of main AC power generation system of the aircraft and the differential protection scheme are discussed． In addition，the effect of differential protection caused by the primary current with DC component，the wrong connection of two phases of CT and the different join order of CT are all considered， which are further discussed by the theoretical analysis and graphical simulation． Finally，Matlab /Simulik simulation results are utilized to validate the correctness of the proposed analysis，and an improved scheme to optimize the effectiveness of differential protection of the system are provided，which can be further used to improve the security of aircraft．