Abstract: In order to solve problems of difficulty in self-development of users and high cost in system development because of using PPI protocol for communication between upper and lower computers in common grouting monitoring system of coal mine, the paper proposed a communication scheme between PLC and upper computer by use of free-port mode. Taking grouting parameter collecting subsystem of grouting monitoring system as an example, the paper detailedly introduced communication principle between upper and lower computers and design scheme of communication software in free-port mode. The actual application showed the communication scheme has quick communication speed, high correctness and stability of data transmission, which provides a good scheme of data monitoring of small and medium sized control system of coal mine.