Abstract: A kind of mine main fan online monitoring system based on Siemens S7-300 series PLC is designed. The controllers and sensors which have high anti-interference, high reliability and are suitable for industrial environment have been adopted. In order to ensure the system’s monitoring values and the fan control instructions can be reliably transferred in the serious electromagnetic interference fan room, the system calculates the exhaust air volume of the main fan through Bernoulli equation to avoid the calculation error which is caused by the duct section uneven airflow.