Wound‐rotor synchronous starter‐generator (WRSSG) has the advantages of less volume and weight, high safety and low cost in maintenance, so it is becoming increasingly popular in modern aircraft, especially for more‐electric aircraft. The main machine (MM) and main exciter (ME) of the aircraft WRSSG are electromechanically coupled seriously, which makes the start control for the WRSSG complex in the starting mode. In this study, a decoupling start control method for aircraft WRSSG based on main field current estimation is proposed to solve the serious coupling problem and improve start control performance. In the proposed decoupling control method, the main field current is estimated first and used in the closed‐loop excitation control for the ME to make the main field current meet the MM demand. With the desired field current, the MM is decoupled with the ME and controlled like a traditional synchronous motor to start the aircraft engine under maximum torque per ampere. The feasibility and effectiveness of the proposed decoupling control method is verified by experimental results.