Abstract: Machine transplanting has been the primary technique of rice seedlings in cultivation in recent years. Among them, factory-scale greenhouses have been commonly used for seedling cultivation in northern China. By contrast, paddy-field seedling can be widely used to cultivate rice transplanting in southern China, according to climate, environment, and production costs. However, manual seedlings are limited to the high labor intensity and the waste of chemical fertilizers and pesticides. It is very necessary to improve the irrigation uniformity in the management of water-fertilizer-pesticide in rice paddy fields. In this study, a variable-rate spraying device was designed for water-fertilizer-pesticide irrigation in rice seedlings. A systematic analysis was performed on the large variable-rate sprinkler and the supporting equipment of fertilizer injection at a high-quality level. The overall structure and working principle were clarified for the variable-rate spraying irrigation device of water-fertilizer-pesticide for seedlings; FLUENT software was used to conduct numerical simulation on the sprinkler irrigation variable-diameter pipeline, in order to determine the optimal structural layout of variable-diameter pipeline; The flow rate of sprinkler and the inlet water of main pipe were then determined after experiments. The curved surface model was established for the pressure and electric ball valve opening; A fertilizer injection and dosing mechanism were designed using proportional integral derivative (PID) self-tuning. The real-time equal-proportion fertilizer or medicine injection was realized after optimization. The online control system of fertilizer injection and variable-rate spraying was constructed with the mitsubishi programmable logic controller (PLC) as the control core. The Box-Behnken experiment was adopted to explore the uniform performance and influencing factors of the device for spraying irrigation. The single objective optimization was applied to determine the key parameters of the device for sprinkler irrigation. The optimal combination of parameters was obtained as follows: The inlet water pressure of the main pipe was 0.20 MPa, the electric ball valve opening was 90°, and the nozzle spraying angle was 80°. Christiansen uniformity of the device was 92.69% in this case; There was a linear model between the mass concentration and electrical conductance (EC) value of the potassium chloride fertilizer solution. The variable-rate irrigation and fertilization experiments were conducted on the device. The EC values of the water-fertilizer mixture were 1.65, 1.66, and 1.68 mS/cm, respectively, in the three spraying irrigation levels. The average values of sprinkler intensity were 900.85, 1092.04, and 1263.67 mm/h, respectively. The fertilization uniformity coefficients were 85.21%, 87.86%, and 91.62%, respectively. There was a small fluctuation of EC values for the water-fertilizer mixtures under the three sprinkler irrigation levels of Ⅰ, Ⅱ, and Ⅲ. The average irrigation intensity of sprinklers varied greatly, where the coefficient of uniformity of fertilizer application was higher than 85%. The field test of rice seedlings was carried out using the variable-rate spraying device of water-fertilizer-pesticide for seedlings. The rice varieties were selected as Huahang51 conventional rice and Guang8You165 hybrid rice. The uniformity of seedling growth was higher than 95% at 13, 21, and 28 d after seeding. There were excellent quality indexes and rooting performance of the seedlings. The various quality indicators of rice seedlings fully met the requirements of machine transplanting. The experiment results showed that the variable-rate spraying device of water-fertilizer-pesticide system shared an accurate, real-time, and proportional fertilizer injection, irrigation, and pesticide spraying in rice paddy fields. The finding can provide a practical application to improve the mechanization level of rice paddy field seedlings for the high-quality of seedlings.