In practical application, some flowmeters are not allowed to be calibrated off-line periodically, thus on-line calibration is necessary to be implemented in workplace . As a non-invasive flowmeter, the clamp-on ultrasonic flowmeters are widely used in on-line calibration system. As a master flowmeter, ultrasonic flowmeters must be calibrated before or during using. Based on multiple fieldbus control system, an automatic calibration system for the ultrasonic flowmeter was designed. The comparison results between ultrasonic and electromagnetic flowmeter obtained during the process of calibrating manifest that the automatic calibration system is effective and reliable. Because of the good interoperability of the multiple fieldbus, other flowmeters with different protocols can be connected into this system easily. With the high speed development of fieldbus technology, this has great practical significance in flowmeter calibration.