Pathologies of the lateral masses could lead to bone destruction of the cervical spine. Their treatment includes lesion resection and fixation. However, the resulting bone defect of a lateral mass is often neglected, resulting in difficulty in bone fusion. Therefore, we designed a subaxial lateral mass prosthesis to achieve lateral mass joint fusion. This study aims to evaluate the role of a new subaxial lateral mass prosthesis using finite element analysis. Five finite element models (intact, lateral mass resection, screw-rod fixation, prosthesis implantation, and prosthesis fusion groups) were compared in terms of the range of motion (ROM), prosthesis von Mises stress, and screw-rod von Mises stress during flexion, extension, lateral bending, and rotation. The ROM of the model increased significantly after lateral mass resection, and was significantly reduced after fixation with screws and rods. Screw-rod fixation combined with prosthesis implantation further reduced the ROM. After bone fusion in the prosthesis, the ROM can also be reduced slightly. The von Mises stress of the bilateral screws and rods significantly decreased after prosthesis implantation. The von Mises stress of the prosthesis further decreased during the right bending after bone fusion was achieved. Subaxial lateral mass prosthesis can help restore the stability of the cervical spine after lateral mass resection and can reduce the stress on the bilateral screws and rods. Reconstruction of a lateral mass is more consistent with the mechanical transmission of the three-column spine and contributes to interfacet fusion of the lateral mass joint.