The tri-eccentric butterfly valve relies on full surface contact between the butterfly plate and the valve seat sealing surface to achieve zero leakage sealing effect. The sealing surface of the processing and manufacturing accuracy has a vital influence on the sealing performance. The existing sealing surface measurement mainly relies on the off-line measurement, and there are some problems such as inconsistent measurement datum and measurement errors caused by secondary clamping. In this study, the precision in-position measurement technique of sealing surface and the original point cloud data processing method is proposed. For the measured sealing surface, the trough-clustering algorithm and the normal vector-surface fitting algorithm considering constraints are proposed. The key parameters and machining error of the sealing surface are obtained. Compared with the least square method, the relative accuracy of this algorithm is improved by more than 60% . The relative error between the cone angle of the sealing surface of the three-eccentric butterfly valve measured in situ and the result measured by the CMM is only 0. 43% , which meets the requirement of ±0. 5% . The measurement accuracy of the in-position measurement technology of the sealing surface has been verified effectively, which provides a reliable technical means for the precision measurement of high-end valves in the future.