When a threephase shortcircuit fault occured on the ship shaft with a brushless doublyfed generator (BDFG), the shortcircuit current would cause a great hazard to the system. Therefore, it was necessary to design a special protection circuit to ensure the safety of the system. Based on the space rotating coordinate system, the dynamic equation of the threephase short circuit on the power winding of the ship shaft with BDFG was established, and the variation characteristics of the system shortcircuit current were studied according to the law of magnetic flux conservation by analyzing the components of the shortcircuit current. It was proposed to add the crowbar resistor and the DC unloading circuit to unload the overcurrent, and the current variation characteristics were further studied after adding the protection circuit. A 30 kVA BDFG was used as the simulation object. Model simulation and theoretical value calculation were carried out by MATLAB/Simulink. The effectiveness of the protection circuit was verified. It was instructive for the electrical safety design of the ship shaft with BDFG.