Abstract: On the basis of analyzing limitation of traditional overload protection of inverse time lag of motor, the paper put forward a new algorithm of overload protection of motor used in converter according to first order equation of thermal model of motor. It introduced methods of parameters settling and amending in the algorithm in details and gave flow of the algorithm. The simulation result showed that the algorithm reflected physical process of heat generation and heat emission of motor in overload process of motor, and preferably solved the problems such as overload fluctuation and intermittent overload that was not solved by traditional algorithm.