The nonuniform coefficient and curvature coefficient are very useful coefficients in soil mechanics. However

the precise approach to calculating

is seldom studied. A modified Van Genuchten (MVG) equation is proposed to fit the particle size distribution curves. Cftool in the scientific programming language MATLAB is applied to fit the function. This paper analyzes the influence of three parameters of

in MVG model on the particle size distribution curves of the thirty-three types of soils

which including the Qinghai loess

the Qinghai relic soil

the Gansu relic soil and the Xinjiang relic soil and so on. It is shown that the fitted curves of MVG equation perform well in predicting the particle size distribution curves

the square of the correlation coefficients are all bigger than 0.98. The parameter a causes the curves to migrate from the left to the right and vice versa

and can also indicate the maximum particle sizes. The parameter m influences the proportion of every group with the different particle sizes and the parameter n affects the slopes of curves. The proposed MVG equation is a precise approach to calculating

.