Nonuniformity in fertilization causes water and soil pollution, thus negatively affecting the yield and quality of crops. Differential pressure tanks are widely used in China. However, such tanks hardly overcome nonuniformity in fertilization. In this study, a flux control method is derived for uniform fertilization on the basis of differential pressure tank with optimal (constant) fertilizer concentration and constant fertigation flux. The fertilizer continuity equation is applied for the flux control method by a uniform distribution assumption of fertilizer concentration in the tank. In accordance with four free parameters, the flux control method is used to divide the constant fertigation flux into two time-dependent fluxes; one flux flows into the tank to remove fertilizers uniformly, and another flux directly flows through the main pipe to mix with outlet fertilizers for an optimal fertilizer concentration in the fertigation system. On the basis of the flux control method, a grade control methodology is further proposed for the convenient and easy division of fertigation flux. In addition, a numerical model for mixed water and fertilizer flow is proposed to verify whether the flux control method is feasible for uniform fertilization. The flux control method is meaningful for improving the management and efficiency for water–fertilizer use in China.