Abstract: In view of problem of distortion of signal of long-distance transmission when using conventional switch quantity, analogue control method to control frequency converter, the paper put forward a method which adopted Modbus protocol to communicate between S7-226 PLC and lenze frequency converter, introduced its communication debugging procedures in details. The actual application showed that the method can effectively resolve problem of signal in distortion process of long-distance transmission, and PLC can easily control running state of frequency converter and read running information of frequency converter under communication mode, so as to achieve purpose of monitoring of running status.