Abstract: In order to obtain an optimal irrigation frequency for watermelon at vegetative period under mulched drip irrigation and deficit irrigation condition in plastic greenhouse, and provide theoretical guidance for the reasonable irrigation of watermelon in arid and semi- arid area in northwest of China, the author adopted‘Nongkeda NO.11’as test material and 0.4 times of pan evaporation (0.4 Ep) as deficit irrigation amount, 3 irrigation frequency treatments were designed, which were T1 (high frequency) of once every 2 days, T2 (intermediate frequency) of once every 4 days and T3 (low frequency) of once every 6 days, respectively. Effects of different frequency treatments under mulched drip irrigation on root zone soil water content, plant growth and related physiological indexes of watermelon were studied. The results showed that the root zone soil moisture of 0-20 cm soil layer under T2 treatment condition was the optimum condition for vegetative period growth, and the level of chlorophyll content, net photosynthetic rate and leaf water use efficiency of T2 were superior to that of other treatments, assimilation capacity of photosynthetic was strong, meanwhile, the watermelon leaves of T2 could maintain an appropriate level of leaf cell sap concentration and intracellular water metabolism. As a whole, under T2 intermediate frequency irrigation treatment, the watermelon plant grew well and the plant was robust at watermelon vegetative period, which had more advantages against adversity stress in early spring. Comprehensively considering soil moisture distribution, physiological and growth index, T2 irrigation frequency of once every 4 days under deficit irrigation condition was the optimum irrigation frequency for watermelon growth at vegetative period in plastic greenhouse at early spring in northwest of China. This research could provide reference for the improvement of deficit irrigation water use efficiency at watermelon vegetative period.