Abstract: At present, in the process of agricultural production, a great use of herbicides have caused the serious pollution of soil environment and the declines of agricultural product quality, and it does not conform to the idea of the sustainable development of agriculture. Variable rate spraying technology is an important development direction to solve this problem. Mostly, the existing variable spraying system changes spraying quantity by adjusting the pressure and by the method of PWM (pulse width modulation) regulating. Pressure regulating mode at the same time of changing nozzle spraying quantity will change the liquid droplets size and spray pattern, and affect the variable spraying operation effect. For the PWM adjustment method, there is the phenomenon that spray status is not continuous at low frequency adjustment, while higher requirements exist for the life and reliability of the electromagnetic valve at high frequency adjustment. And its cost also increases accordingly. The present design of variable spraying system also has the disadvantages of high cost. In order to overcome the shortcomings of existing variable spraying system, this paper designed variable system based on the combined nozzle spraying and had corresponding test analysis. Firstly, the variable-spraying affecting factors model was established. On this basis, this paper designed the variable spraying system of multiple nozzle combination, with single ridge corresponding to the combination of 3 nozzles. High-performance electromagnetic valves were installed in front of each nozzle. By means of controlling the open-close combination of three-way electromagnetic valves, 7 kinds of nozzle spraying combinations were formed. Finally, it achieved 7 kinds of spraying amounts. The designed system could not only control the overall amount of spraying, but also increase regulating range of spraying quantity. In this paper, we designed the pipe of variable spraying system. It mainly included spraying box, strainer, diaphragm pump, safety valve, spraying divider, drip-proof nozzle, pipeline, and so on. Mathematical model of spraying network was established by using fluid network theory; and we analyzed the flow resistance of the system. In this paper, using machine vision system for collection and analysis of field crops, we made out the distribution level of weeds through the analysis of the weed ratio in the field crop with controlling the corresponding nozzle on or off. Such a design method not only achieved the function that system could do variable spraying according to the actual distribution of weed, but also greatly improved the image processing speed of the system, which enabled the whole system to achieve real-time collecting and processing. When variable spraying system was working, host system collected data, and then it disposed the distribution information of weeds, and passed the decision-making information after treating to MCU. Finally, MCU processing decision-making information was used to control the electromagnetic valve of switch state combination, and switch state combination of nozzle was controlled. Different combinations were formed by different size of nozzle in order to realize the function of variable spraying. At the same time, in the process of operation, the information feedback module was designed for the system such as Holzer flow, speed, pressure and liquid level. The liquid crystal display module could display spraying system working condition in real time. Also, in the process of system designing and debugging, we adopted the process of modular design. In this way, it not only simplified the debugging workload, but also made it easier to conduct the maintenance and upgrade of programs. In the end, in order to verify the feasibility and validity of the variable spraying system of multiple nozzle combination, this experiment was carried out on the spray performance comprehensive test bench. By analyzing the test data, it indicates that variable spraying system of multiple nozzle combination is feasible. Each mode spraying had the theoretical value error of less than 10%, and variable spraying system had a wide adjustable range, greatly decreasing the amount of pesticide spraying. In conclusion, the variable spraying system is simple to operate and has low cost, and it is easy to promote. This design can save pesticide and protect the ecological environment, with other multiple benefits for economy and environment.