The invention discloses a locking ring auxiliary assembling device and an engine power turbine assembling method with a locking ring. The locking ring auxiliary assembling device comprises a body withan accommodating cavity, a first fixed edge and a second fixed edge; the first fixed edge and the second fixed edge are fixed to the body; the first fixed edge is fixed to the end surface of the first end of the body and is provided with lugs extending outwards in a radial direction; the multiple lugs are arranged in a circumferential direction of the body and correspond to retaining ring grooves; the second fixed edge is fixed in an accommodating inner cavity of the second end of the body and is provided with grooves which are sunken inwards in the radial direction; the multiple grooves arearranged in the circumferential direction of the body and correspond to fixed claws; and the inside diameter of the body is larger than or equal to the outside diameter of a rotor to enable the body to be in clearance fit to the rotor. According to the technical scheme of the invention, alignment and assembly of the locking ring and the rotor can be finished with single heating of the locking ring, the assembling efficiency is high, repeated heating and plugging and disassembly of the locking ring are not needed, and the whole assembly process is small in damage to the locking ring.