Abstract: The paper proposed a design scheme of Modbus RTU communication based on CP341,gave the selection and configuration of hardware and software,and introduced the design of data communication program between CP341 and intelligent sensor. The scheme uses Step 7 program to realize Modbus RTU communication between S7-300 PLC and intelligent sensor on the basis of ASCII communication mode of serial communication module of S7-300 PLC CP314,and resolves the key technologies such as analysis of Modbus RTU communication protocol of intelligent sensor,implementation of CRC-16 cyclic redundancy check in S7-300 PLC,sending of query instruction,and analysis and storage of receiving data packet.