A watermelon cultivation of intelligent irrigation system was designed based on ZigBee wireless sensor network. The system is composed of environment information acquisition device, information processing system and irrigation implementation structure. The environmental information collection device collects the substrate relative humidity and air relative humidity of the watermelon moisture demand, and analyzes and processes the data in the information processing system to judge whether the irrigating agency is driving the irrigation. Considering the influence of substrate relative humidity and air relative humidity on the water demand of watermelon at different growth stages, a fuzzy control rule was established to test the validity by MATLAB simulation. The results showed that the intelligent irrigation system could be irrigated according to the intelligent irrigation strategy, and the substrate relative humidity of the substrate was compatible with the growth stage of watermelon. The substrate relative humidity was controlled at 62.82%–67.25% during the period of sprouting stage. The substrate relative humidity was controlled at 68.05%–72.18% during the period of flowering stage. The substrate relative humidity was controlled at 72.23%–77.15%during the period of expanding stage. The substrate relative humidity was controlled at 58.36%–62.13% during the period of maturing stage. An error of 2% was obtained between the measured optimum substrate relative humidity at a substrate relative humidity, which meet the needs of watermelon cultivation substrate.