The self-locking gear has great potential application in controlling the position stability of gearbox, which is a critical requirement in some precision machineries and instruments. This study provides important knowledge about the dynamic performance of self-locking gear pairs. An analytical model of variation ratio of contact length (VRCL) was established. The tooth root stress, bearing force, and axial acceleration of three self-locking gear pairs are investigated by using transient dynamic finite element analysis (FEA). The FEA results presented the influences of VRCL on the meshing performance of self-locking gear pairs. The obtained results provide significant knowledge for predicting the dynamic performance of self-locking gear pairs, optimizing their design parameters, and diagnosing possible design errors in self-locking gear pair design.