In this paper,the physical base of the mean square deflection method extracting information of multi-parameter earthquake precursors has been studied.The physical base of this method is that rock properties including elasticity,deformational property,electric property,magnetic property,thermal property and so on will change obviously when crushing stress reaches up to more than 50 percent during earthquake preparation,which results in changes of absolute value,variation rate and measuring error of rock physical parameters.To calculate the changes by using the mean square deflection method,on one hand,eliminates the seasonal interference of precursor parameters,on the other hand,gives prominence to the precursory informations.