For the problem that power circulation and low efficiency easily occur in closed Continuously Variable Transmission (CVT), the input-coupled CVT drive system was taken as the object of study, the fundamental structure and working principle of the input-coupled CVT drive system were briefly introduced. In the circumstances of no energy loss, the sufficient condition for the input-coupled CVT drive system to realize power split was deduced in detail. Next, the meshing efficiency expression for the 2K-H type differential gear train was simplified under the sufficient condition. The condition for maximizing the meshing efficiency of the 2K-H type differential gear train was demonstrated. Finally, the conclusion was verified by example. The results show that the condition of maximum meshing efficiency ensures the highest meshing efficiency of the 2K-H type differential gear train from the design point of view.