The invention discloses a car electric power-assisted steering system, a thermal protection control method thereof and a car. The electric power-assisted steering system comprises a mechanical steering assembly, a speed reducing mechanism connected with the mechanical steering assembly, an assist motor, a first temperature detection module used for detecting the temperature of a carbon brush of the assist motor, and a controller, wherein the controller comprises a driving H bridge, a second temperature detection module and a control module; the second temperature detection module is used for detecting the temperature of a switching tube of the driving H bridge; and the control module controls the working current of the assist motor through the driving H bridge according to the temperature of the carbon brush of the assist motor and the temperature of the switching tube of the driving H bridge. Through the system, the situation that the carbon brush of the assist motor and the switching tube of the driving H bridge work abnormally due to long-time working under large current can be avoided while the car electric power-assisted steering function is ensured, so that the service life of the electric power-assisted steering system is prolonged, and the reliability and the stability of the electric power-assisted steering system are ensured.