We calculated the barometric efficiency and correlation coefficient of discharge changes of Ligou well, Qingshui, and extracted abnormal changes before medium and strong earthquakes since 1985 after excluding interfering factors of rainfall and human activities. The results suggest that anomalies of barometric efficiency often appear before most of medium and strong earthquakes in Qilian seismic zone and those of correlation coefficient also appear before some earthquakes. So we established an earthquake prediction mode which was fit for the well through studying multiple linear regression model, and it may provide a new method for quantitative prediction of three elements of earthquakes.