As the precise sensor system for monitoring the relative difference in height among multiple points, the hydrostatic leveling system (HLS) is widely used in particle accelerators. At present, the HLS system is mainly used for the measurement of relative altitude changes. To expand the application of HLS system in accelerator alignment, the research on the issue of HLS system for altitude difference measurement based on the horizontal plane between multiple points is carried out. Firstly, the measurement principle of capacitive HLS sensor is introduced briefly. Based on the working principle of the sensor, a comparison system composed of dual-frequency laser interferometer, high-precision Z stage, HLS sensors and others is designed and manufactured. The system is used to control multiple sensors to observe the same liquid level in the same coordinate system. The zero-position difference between multiple sensors based on the sensor coordinate system are obtained by comparison, and the absolute height difference measurement with respect to a hydrostatic surface between the multi-sensor coordinate systems is realized, and it is verified that the measurement accuracy is better than 5 μm. In addition, by installing the target holder above the HLS sensor, and using the coordinate measuring machine (CMM) to strictly calibrate the distance between the sensor surface to the center of the target of each sensor, the absolute height difference measurement of the position of the multi-target centers with respect to the hydrostatic surface is realized, and its measurement accuracy is better than 30 μm.