In this paper, a method of sensor less best exciting in towards polarity by stator-electricity of synchronous motor is presented on the base of calculated best exciting time through simulation in order to solve the present problems and enhance the reliability of excitation exist in asynchronous starting of synchronous electrical machine. Detect and analyze the wave of stator current to catch the best excitation time accurately to realize best excitation. The results of experiment demonstrated the advantages of high reliability, short time and low impact when pulling into step, much easier to pull into step in low voltage and heavy load condition, which showed the feasibility of this method.