Aiming at the causes generating differential deviation and possible potential consequences in the statistics model of dam safety monitoring, two methods used for detecting differential deviation,i.e.,graphic method and Glejser method are presented. The improvement to the statistics model causing differential deviation is performed by using stable transformation method and weighted regression estimation method so as to decrease the influence of differential deviation on model parameters estimated values,and so the regression model can better fit observed date and raise the reliability of predication and control.