Abstract: The finding of dispersion parameter is a prerequisite to recognize correctly the pollutant migration of underground water and its control.Based on the extremun theory and linearisation of nonlinear equation,the linear relationship between concentration and time variation is deduced,the linear graphic method used to discriminate the dispersion parameter is obtained.It can remedy the insufficiency of other methods.It is checked with the help of practical dispersion test data and is indicated that the established mathematical model is coincided better with the practical model,that is,the obtained parameter is confident.