Abstract: The transmission system of the transverse Dual-Clutch Transmission is very complex, so the gear ratios con not be defined quickly only according to the theoretical method. Based on the characteristic of transverse Dual clutch transmission system, we analyze the influence of transmission structure, relationship of gear meshing, axis center distances, and provide a definition method of gear ratio which can both follow the theoretical requirements and meet the limitation of actual structure factors. The practical application shows that this method is feasible and can set the actual speed ratio in the complex structure of the transverse double-clutch transmission., And the matching vehicle can meet the requirements of dynamic and economic performance.