Abstract: 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 continuous 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.