The invention provides a boat helm / wing rudder discretional rotation angle transmission device. The invention consists of a main rudder blade, a main rudderpost, a photo-electricity encoder, a wing rudder leaf, a wing rudder shaft, a wing rudder driven gear, an internal bearing, a turbine sleeve shaft, an external bearing, a worm motor component, an external base seat, a gear bearing, a bottom seat, a main shaft drive gear, a middle supporting plate, an upper base seat, a harmonic reducer, an input shaft sleeve, an upper cover, a top motor supporting cover, a main shaft motor, a top encoder supporting cover, an encoder coupler, a photo-electricity encoder, a main shaft driven gear, a frame, etc. The invention enables the rotation of the wing rudder out of the restriction of the main rudder, namely, a small control surface which correspondingly and independently motions is added on the rudder to obviously improve the fluid power performance of the rudder, thus improving the course control performance of the rudder system. The invention has the advantages of reducing the energy consumption, improving the reliability and improving the navigability of the boat.