The invention relates to a steering gear, in particular to an underwater electric steering engine and a rudder angle detection method. The underwater electric steering engine comprises an output shaft, an engine base, a worm wheel, a worm, a power system and a steering engine controller. The power system drives the output shaft to rotate through transmission of the worm and the worm wheel. The output shaft extends out of the end, provided with a shaft end cap, of the engine base, and the output shaft is provided with a limiting swing piece at the end. A swing groove is formed in the shaft end cap. The limiting swing piece swings in the swing groove. The steering engine controller is connected with a motor in the power system. An incremental encoder is arranged on the motor. The limiting swing piece and the steering engine controller are used for determining the rudder angle zero point position. The incremental encoder is used for detecting angle change values of a motor output shaft so as to determine the absolute position of the current rudder angle. The power system is arranged in a shell. The shell communicates with the engine base, and the interior of the shell is an oil-filled sealed space. An oil pipe is connected with the shell. The underwater electric steering engine is compact in overall structure and capable of meeting operation demands of full-ocean-depth low-speed large-torque and large-moment output.