To improve the contact conditions in torque-split systems, this study proposes a contact pattern optimization method for face gear pairs, with alignment installation errors as boundary conditions. Firstly, we establish a finite element model of the torque-split transmission system. The objective is to extract the deformation at the face gear web and the inner/outer gear rings of the small gear under multiple operating conditions and then equate this deformation to the alignment installation error of a single pair of face gears. With tooth contact ratio and transmission error amplitude as optimization objectives, tooth surface modification parameters as design variables, and alignment installation errors as boundary constraints, we conduct contact pattern optimization for each branch face gear pair. Finally, a modification scheme for the face gear torque-split transmission system that meets practical requirements is obtained.