Abstract: Hydrostatic Levelling System (HLS) with intellectual property was researched, which would be used in real time supervising the vertical position of storage ring and key components. The paper introduces the practical application of the HLS in the pre-alignment of BEPCII. By analysing the data, it is sure that the HLS can work smoothly and the precision and other technological parameters conform to requirement. The data also provided a good reference for the surveying, assembling and testing of the parts used in BEPCII. The defense methods of the system in the process of practical using are introduced too.