The aquifer can be treated as poroelastic media in current research. An ideal 2D confined aquifer model was founded to calculate the response of the groundwater level to the variation of stress field. Then the dissipation law of abnormity of groundwater level was discussed. A deconvolution method to invert stress-strain field from groundwater level data when considering the dissipation process was also discussed. Results from forward calculation and earth tide data of Huili Chuan-06 well were analyzed to confirm the deconvolution method. At last, the mechanism of co-seismic step variation in groundwater level was analyzed. It was concluded that a simple poroelastic model cannot explain the amplitude of variation.