A design scheme of triple-redundancy transmission systems was presented in which three groups of power units parallel input and backup each other. A compound gear train transmission system coupling with input movement and power makes sure continu-ous power output by automatically switching to another mode while part of the input unit fails. Based on the analysis of the design scheme, a structure design scheme was introduced, and the matching relationship of gear teeth was derived to meet the requirement of transmission ratio. Then a multidisciplinary design optimization model of the triple-redundancy transmission system was established, in which the system is discomposed into two subsystems: differential gear train and fixed axis gear train. The multidisciplinary variable couple design optimization method of discrete variables was used in the optimal design of the triple-redundancy transmission system and the optimal comprehensive scheme was obtained which meets the matching relationship of gear teeth, strength requirements and kinetic requirements.