The invention provides a method for positioning longitudinal osteotomy position of tibia in unicompartmental knee arthroplasty, an intramedullary positioning rod and a thighbone drilling guide auxiliary device are used for determining the drilling position of a fixing hole used for mounting a thighbone prosthesis on the thighbone, the fixing hole is prepared under the guiding effect of the thighbone drilling guide auxiliary device, the postartis of the thighbone is cut off, the meniscus is cut off, a thighbone test mold with the appropriate thickness of the distal condyle is selected and installed after bone surface primary forming, the knee joint is bent and stretched repeatedly, and the longitudinal osteotomy position of the tibia is determined through the thighbone test mold. By capturing the motion trail of the tibia joint during relative normal motion, reference is provided for longitudinal osteotomy of the tibia platform, so that the longitudinal osteotomy position of the tibia is determined, the rotation alignment of the tibia prosthesis is closer to the motion trail of the tibia joint, collision between the prostheses is avoided, the motion compatibility and coincidence of the thighbone and tibia prostheses are better, the postartis of the thighbone is preferentially cut off, so that the meniscus on the tibial plateau is cut off more easily, and the collateral ligament is prevented from being damaged.