By adopting the fuzzy control theory, a greenhouse irrigation automatic control system has been designed with the characteristic of accurate of the nonlinear, time-variability, long time delay and the characteristic of accurate mathematic model being hard to establish. Through some basic experiments, the basic domain of input and output of the fuzzy controller were ascertained. Time variables that were used to control the each connect duration of electromagnetic relay. Fuzzy control can be realized by MSP430F133 MCU control system with usage of the time variable. By the action of fuzzy control, prototype field experimental results showed that first-order inertial character performed by the system with the constant —3540s, which was no overshoot. With simple structure, low price and high reliability, the accuracy of the system meets irrigation technical requirements of crops in greenhouse.