The theory and method of the loading and unloading response ratio to the earth tide have been quoted in data calculation of tides of the gruondwater level.The feasibility for calculating the loading and unloading response ratio by means of the observation data of the gruondwater level is proved, and the calculation method has been showed.The loading and unloading response ratio of the groundwater level to the earth tide in the North China during the last 20 years has been calculated by this method, and the variation of loading and unloading response ratio of groundwater level to earth tides before and after the Datong M_S6.1 earthquake in 1989 has been analyzed.Increasing anomaly variation of the loading and unloading response ratio of most of the well levels to the earth tides appeared from 6 to 12 monthes before the earthquake.