For the dual- cavity half toroidal continuously variable transmission mechanism composed of half toroidal continuously variable unit（ CVU）,the fixed- axis gear train and the double- row differential gear train,the condition of the transmission ratio of the fixed- axis gear train and the speed ratio of the shifting mechanism on the double- row differential gear train must meet are analyzed under high torque transmission and power split. The expressions for the ratio of the power through half toroidal CVU to the input power and for the transmission efficiency of the mechanism are derived under power split. Whether the transmission ratio of the fixed- axis gear train decreases or the speed ratio of the shifting mechanism on the double- row differential gear train reduces,the ratio value of the power through half toroidal CVU to the input power reduce and the transmission efficiency increases. Finally,the example demonstrated the influence of the transmission ratio change of the fixed- axis gear train and the speed ratio change of the shifting mechanism of the double- row differential gear train on the ratio value of the power through half toroidal CVU to the input power and the transmission efficiency of the mechanism.