In view of the working car's lifting window produces heat can destroy the motor of lifting window, a software thermal protection method of motor for car lifting window is proposed. Firstly,through analyzing the theory of permanent magnet DC motor's temperature variation under three working states, the mathematic models of thermal protection algorithm are built, and the real-time monitor software of motor's temperature is designed. Secondly, the experimental verification of software monitoring method is carried on under voltage of 12.5V and 14.5V. At last, the result shows the error between monitor temperature and actual temperature is less than 5℃, which means this method has high accuracy in monitoring of motor's temperature variation, and can realize thermal protection for the motor of car's lifting window. It will be applied in the field of motor thermal protection without temperature sensor.