Abstract:“Joule balance method” is a Planck constant measurement method based on mechanical electromagnetic energy balance. Due to the existence of a “feature vector alignment energy error” caused by the nonalignment state of the coil feature vector, the measurement uncertainty of the Planck constant can hardly be reduced down to 1×10-8. In order to realize the accurate compensation of the alignment energy error, aiming at the precise measurement issue of the residual horizontal displacement during the vertical movement process of the energy balance coil, a differential linesurface displacement sensing measurement method based on the electromagnetic selfdamping direct vertical measurement reference is proposed in this paper. In order to avoid the introduction of the system error in the adjustment process of indirect measurement reference, which leads to the reducing of the measurement accuracy, the linesurface displacement sensor uses a thin wire electrode as the direct vertical measurement reference. The differential capacitance structure of three electrodes is designed to inhabit the nonlinearity of the singleend capacitor. The surface shape of the other two electrodes is designed as mutual opposite outer cylinder, which uses the geometric shape isotropy of the cylinder surface in the cross section to offset the influence of attitude tilt on the measurement results, so as to improve the adaptability of the sensor to complex measurement conditions. And an electromagnetic damper is used to stabilize the thin wire electrode and ensure the accuracy of the measurement results. The experiment results show that the synthetic extended uncertainty of the differential selfdamping displacement sensor is about 281 μm within the normal working range.