In this paper,the lower boundary flow of considered upper soil stratum was expressed into linear relation with infiltration ratio and saturated degree by introducing preferential flow coefficient.So according to principle of salt-water balance,the water deposit formula and salt deposit formula was conducted out by introducing leaching coefficient.Through discussing above formulas,the critical mineralization degree equation of irrigation water was presented.The physical significance and af ected factors of the preferential flow coefficient and leaching coefficient were discussed.The salt-water regime model is simple and suitable for farmland irrigation management,the critical mineralization degree equation of irrigation water maybe provide reference for option of mineralization degree of irrigation water.