The waterfertilizer mixing process with an opentank in the fertigation system was analyzed by a simplified dynamic mixing process in this paper. Then a variable universe fuzzyPI control strategy was proposed which aimed at the characteristics of nonlinearity, pure hysteresis, time degeneration and uncertainty in the control system. A set of stretching factors with proportional exponential function was used to modify the universe of the fuzzy controller on line for improving its adjustability. Considering the influence of varied rotational irrigation schedule, a cofuzzy controller with onedimensional was proposed to regulate the reference value of integral coefficient of PI controller, through monitoring the system flow change on line. And the correction value of control parameters of PI controller were regulated by the variable universe fuzzy controller. In order to verify the validity and advantage of proposed control algorithm(VFPI), four experiments with different recipes and three experiments with different rotational irrigation situations that compared to the traditional PI controller were carried out, while the testing platform of fertigation control system was established. The contrast testing results demonstrate that the proposed VFPI control strategy has better adjustability than the traditional PI control method. When the recipe contents or rotational irrigation situations varied, the VFPI controller has smaller overshoot, better dynamic quality and reliability than the traditional PI controller.