In order to solve the technical problem of evaluating the accuracy of the open channel flowmeter in the on-line wastewater automatic monitoring system, a set of on-line calibration device of the open channel flowmeter with high-precision pipeline flowmeter as the standard meter is designed. A special on-site flow signal acquisition and measurement system is developed. The hardware circuit is designed, and the matching upper computer operation software is developed by QT. Through a large number of field comparative tests under different conditions, the reliability of the system and the effectiveness of the device are verified, providing a technical basis for the accurate online monitoring of wastewater flow.