The traditional process of injection molding requires the frequency converter to quickly switch speed in order to meet the pressure and flow rate that the system needs, and the generated impulse current is absorbed through increasing the capacity of the frequency converter. A synchronous controller was introduced which could change the output of the frequency converter in advance according to the process signal, thus buffer time was provided for its speed changing, and the frequency converter was protected and its capacity was made match with the capacity that the system needs. The composition and working principle of the synchronous controller were illustrated in detail. Practice shows that operation of the synchronization controller is stable, reliable and it effectively reduces the impulse current.