The invention relates to a thermostat of an engine cooling system, in particular to a variable-runner thermostat driven by a motor. The variable-runner thermostat comprises a valve body with an inner cavity. A water inlet communicating with a water jacket water outlet pipe is formed in the right side of the valve body, a first outlet connected with a water pump water supply pipe is formed in the lower portion of the valve body, and a second water outlet connected with a heat radiator water supply pipe is formed in the upper portion of the valve body. A variable-runner baffle is arranged in the inner cavity and connected with the motor through a center shaft. One half side of the variable-runner baffle rotates between the two sides of the first outlet, the other corresponding half side of the variable-runner baffle rotates between the two sides of the second water outlet, and the rotating angles of the variable-runner baffle are controlled by the motor. The variable-runner thermostat is driven by the motor, and when the work condition of an engine changes, the response is fast. Different operating strategies can be adopted according to the change of the work condition of the engine to improve the performance of the engine. Finally, due to the controllable flow area, different circulating cooling water circulating amounts can be precisely controlled, and the cooling system can be better optimized.