The invention provides an electric-assistance variable nozzle turbocharging system, and belongs to the technical field of turbochargers. The electric-assistance variable nozzle turbocharging system and the control method thereof aim at enabling a motor and a turbine variable nozzle ring to be coordinated through the influences of the motor and the turbine variable nozzle ring on performance of an engine. A diesel engine cylinder is directly connected between a gas inlet branch pipe and an exhaust branch pipe of the system. A gas inlet pipe is connected to the gas inlet branch pipe. An exhaust pipe is connected to the exhaust branch pipe. An intercooler is installed on the gas inlet pipe. An electric-assistance variable nozzle turbocharger is installed at an inlet of the gas inlet pipe and an outlet of the exhaust pipe. According to the electric-assistance variable nozzle turbocharging system and the control method, the high-speed motor is controlled on the basis of the air-fuel ratio of a diesel engine and the state of a storage battery, the gas inflow amount of the cylinder can be effectively and rapidly regulated so as to be matched with the changes of the oil spray amount, coordinated synchronization of oil and gas is achieved, it is ensured that the diesel engine keeps an appropriate air-fuel ratio value under various working conditions, and meanwhile it is ensured that the battery works in a high-efficiency area.