In order to solve the present problems and enhance the reliability of exitation exist in asynchronous starting of synchronous electrical machine, a method of sensorless best exciting in towards polarity by stator-electricity of synchronous motor is presented on the base of calculated best exciting time through simulation. Detect and analyze the wave of stator current to catch the best exitation time accurately to realize best exitation. 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.