The invention provides a dual-motor coupling electric power-assisted steering device and a control method thereof. Mode switch is achieved by adopting a magnetic powder clutch and a synchronizer, and a single-row planetary gear system serves as a power coupling device of a first motor and a second motor. A first clutch and a second clutch limit torque output of the first motor and the second motor correspondingly, and the energy flow direction of the second motor is changed through the synchronizer. According to electric current on-off control of an ECU over the clutches, power output of the motors is determined. By using joint conditions of a joint sleeve and combing on-off of the clutches, coupling conditions are determined. According to motor failure and a drive circuit self-diagnosis technology, the power-assisted steering module ECU judges the states of the motors. By controlling the clutches and the joint sleeve, dual-motor fault-tolerant control is achieved. According to the dual-motor coupling electric power-assisted steering device and the control method thereof, four working modes including dual-motor independent drive, dual-motor rotating-speed coupling drive, dual-motor torque coupling drive and the like can be achieved, requirements for various performance of steering in the driving process of a commercial vehicle can be met, and a system is high in reliability.