TI worm drives are widely used in the machinery industry due to their advantages of large transmission ratio and high load carrying capacity. In this paper, the contact line is discretized based on the gear meshing principle, and the stiffness is calculated by combining with the gear cantilever beam model, thus forming the stiffness matrix. The contact force curves are obtained by constant iteration of the deformation, and the transmission efficiency of the TI worm drive is calculated. The results show that the theoretical and finite element trends of contact force changes as well as the theoretical calculated efficiency and experimental efficiency change curves are consistent. However, the meshing impact is not considered in the theoretical calculation, which leads to the inability to effectively predict the fluctuation of the curve. And the theoretical average efficiency is larger than the experimental. However, the error range is less than 10%, which indicates that the theoretical model is able to predict the contact force distribution law and efficiency change of the TI worm drives. It provides a simpler and faster calculation method for TI worm drives in industrial applications.