In order to improve the fuel economy of hybrid electric vehicles, a dynamic coupling mechanism of five rod planetary gear train was designed, using double planetary rows to couple three power sources, in which the engine was connected to the planet frame, the motor 1 was connected to the sun wheel 1 by the sprocket 1, and the motor 2 was connected to the sun wheel 2. The power was transferred to driving bridge through the sprocket 2. The relationship between the rotational speed of the output shaft and that of three power sources were derived. The analysis of transmission ratio showed that this mechanism could realize continuously variable transmission in operation while achieving dynamic coupling. The energy flow analysis was carried out under the operating conditions of engine launching, starting, accelerating and uphill, cruising, skidding, braking, low speed pure electric power and backing-up. The Advisor software was used to verify the effects of this coupling mechanism in a model vehicle under the new European driving condition. The results showed that the recovery of braking energy was increased by 45 kJ, and the recovery rate was increased by 3.42%. The fuel economy of the hybrid electric vehicle was improved effectively.