Taking the main beam type TBM propulsion mechanism as the research object, the inverse kinematics equation and Jacobian matrix were obtained by using coordinate transformation theory. The force of the driving mechanism in the process of straight line driving was analyzed and the force balance equation was obtained. In order to reduce the damage of partial load to the mechanism and cutter head, the force distribution evaluation index was introduced, and the relationship between different parameters and the force distribution index was analyzed. Aiming at minimizing the force distribution index, an optimization model of the mechanism’s scale parameters was established, and the genetic algorithm was used to solve the problem. An example of scale optimization design of the mechanism was given, and the change of maximum joint force before and after optimization was compared. The results show that the optimization model is reasonable and effective, and the relevant results can provide reference for the design of the propulsion mechanism.