Aiming at the problem of how to ensure the measurement accuracy of the on-site mounted ultrasonic flow measurement device, a precise measurement method of geometric parameters of the on-site mounted probe based on articulated arm coordinate measuring machine(AACMM) is proposed. Firstly, the flow-rate calculation model of ultrasonic flow measurement device is analyzed, the results show that the measurement accuracy is based on the accurate measurement of the geometric parameters of the probes. On this basis, through special design of probe and comparison of internal and external test, feasibility of using external probe 3-D coordinates value measured by AACMM to calculate probe actual geometric parameters is verified. Finally, in three test cases of middle route of south-tonorth water transfer project, 3-D coordinates value of probe of plug-in four-path ultrasonic flow measurement device are accurately measured by AACMM, precise geometric parameters of the probe and corrected weight coefficients are obtained by analysis. Through analysis of the influence of different weight coefficients on the average flow velocity of ultrasonic flow measurement device, the results show that the method can improve the measurement accuracy of the on-site mounted ultrasonic flow measurement device by 0. 02% ~ 0. 58%, the accuracy of scribe localization is evaluated, and also the rationality and effectiveness of the method are verified.