To fully realize the advantages of AC-AC variable frequency speed regulation performance, based on the principle of pulse blocking AC-AC frequency conversion under the traditional voltage balance, a three-phase pulse blocking type AC-AC is proposed when the input voltage is unbalanced. Through the theoretical analysis of the chopping control after blocking, fully-controlled power devices are used, and the duty cycle expression corresponding to each interval is derived according to the area equivalent principle. The frequency conversion amplitude modulation of the output voltage under the condition of unbalanced input voltage is realized, which make the frequency conversion speed control system achieved the running effect when it is in equilibrium. Finally, the mathematical model of three-phase pulse blocking type variable frequency speed control system with unbalanced input voltage is established, and a semi-physical simulation platform is designed. The simulation results are consistent with the results under equilibrium conditions, which proves the feasibility of theoretical analysis.