The invention relates to a hybrid electric vehicle series-parallel type double-planetary-gear-train dynamic coupling device and method. The dynamic coupling device comprises an engine, a first planetary gear train, a second planetary gear train, a first motor and a second motor. The first planetary gear train is composed of a first sun gear, a first planetary gear carrier, a first gear ring and a planetary gear of the first gear ring. The second planetary gear train is composed of a second sun gear, a second planetary gear carrier, a second gear ring and a planetary gear of the second gear ring. The first motor is connected with the first sun wheel. The second motor is connected with the second sun wheel. The engine is connected with the first gear ring through a clutch. The first gear ring is provided with a brake. The first planetary gear carrier and the second gear ring are fixedly connected together to serve as the power output end. The second gear ring is connected with an outer transmission mechanism through a power output gear. The second planetary gear carrier is fixed to a frame. The dynamic coupling device is simple in structure, easy and convenient to control, and capable of switching various dynamic coupling actions and modes, improving the oil consumption characteristic and the power performance and achieving stepless speed change, and a transmission does not need to be additionally arranged.