Abstract: It is suggested that the zero hydrostatic pressure level (or plane) should be defined as the groundwater level (or plane). Six factors for the groundwater measurement are analyzed. At the same time, the necessary and sufficient conditions for the decrease and increase of groundwater level are also shown. By analyzing the vacuum preloading mechanism and system of forces, necessary and sufficient conditions for change of groundwater level and field test results of groundwater level, the fact of the soils that are still saturated after improvement, and field data indicate that there are no lowering of the groundwater and preloading. It is illuminated that the groundwater level does not owing to ground improvement by vacuum preloading. A new type of rod instrument is put forward to measure the groundwater level. It is a direct method to measure the groundwater level, which eliminates the effects of incorrect vacuum degree and internal tube leaking as measured by means of the double pipe method. So this new instrument is reliable to measure the groundwater level.