Objective To investigate the biomechanical effects of the backside design of tibial trays on the bone-prosthesis fixation interfaces in unicompartmental knee arthroplasty (UKA). Methods Finite element models of medial knee arthroplasty were constructed using a fixed UKA prosthesis. The knee joint load and joint motion under walking motion were considered as boundary conditions, and the differences in tibial von Mises stress, contact stress, and micromotion of the bone-prosthesis fixation interface of the UKA tibial trays with big keel, small keel, two-peg with fin, three-oblique peg, and three-upright peg types were compared. Results At the maximum medical knee force moment, compared to the two-peg with fin type, the tibial von Mises stress, contact stress, and micromotion of the bone-prosthesis fixation interface decreased by 8% and 15.9% and increased by 9.9% for the big keel type; decreased by 12.3% and increased by 7.5% and 0.9% for the small keel type; decreased by 10%, 10.5%, and increased by 1.2% for the three-oblique peg type; and decreased by 7.7%, 14.7%, and 1.6% for the three-upright peg type, respectively. However, the maximum micromotion of the bone-prosthesis fixation interface occurred at 21% of the gait cycle. Compared to the two-peg with fin type, the micromotion of the bone-prosthesis fixation interface increased by 11.6% for the big keel type, increased by 1.6% for the small keel type, decreased by 0.4% for the three-oblique peg type, and decreased by 2.3% for the three-upright peg type. Conclusions To improve the long-term fixation effects of tibial prostheses, it is recommended to focus on a two-upright peg with fin or small keel designs when UKA tibial trays are designed, which can effectively balance the stress transfer and interface micromotion, thereby ensuring prosthesis stability and reducing the risk of aseptic loosening.