Riser, which is a prerequisite for deepwater drilling, is connected to the sea platforms and subsea wellhead choke. However, the riser in the role of ocean currents can easily produce fatigue damage. Therefore, for the safety of riser, it is essential to monitor the ocean currents which across the riser during drilling. In this paper, a new method was proposed, which put some ADCP points along the outer wall covering of riser to measure flow rate according to certain rules. However, when using this method to measure flow rate, riser motion introduce measurement errors to the flow measurement device. To this end, this paper analyzes the causes of errors generated. Second, expand the theoretical analysis, the results from the analysis showed that ADCP flow measurement error caused by riser vibration could neglect. Flow measurement method that described in this paper is also feasible.