This paper puts forward a new discrete frequency control method of induction motor based on thyristor control, the hexagon flux linkage consisting of voltage space vector is triggered in multiple power cycle, and the hexagon flux linkage locus control of induction motor is realized. Through adjusting the trigger sequence of trigger angle and vector, the magnitude and direction of flux linkage are changed, stable motor rotating magnetic field is formed. By changing the voltage vector of the trigger interval, the motor air gap magnetic field average rotation speed can be changed, and the induction motor with the variable frequency control is realized. After the analysis of the four frequency dividing control process, a transient mathematical model of two-phase and three-phase to two-phase conduction is set up, the relationship between stator current and trigger angle is received. In the method of discrete frequency, the problem of open circuit zero voltage vector is proposed. Theoretical analysis and simulation experiments show that when the discrete frequency control based on flux linkage locus is used in the motor starting, the torque is improved and the current is reduced effectively. According to the control method and the mathematical model, simulation and experiment are made and the consistent results are obtained.