Abstract:According to the understanding of JJG711-1990 and the actual use in the field, the on-line calibration process of rectangular thin-wall weir open channel flowmeter is proposed, the flow indication error is calculated and the uncertainty is analyzed, so as to provide reference for its application promotion and measurement performance research. From the results, the discharge coefficient is the main source of the uncertainty of the discharge error and the way of measuring the water level directly on the water surface makes the flow fluctuation have a great impact on the discharge measurement error.