In order to reduce herbicide dosage and automatic precise weeding in cotton fields，a variable target weed spraying system uses fuzzy control algorithm with processor core of S3C2410 was designed. This system achieved automatic weeds identification, system speed and flow information collection, flow and pressure adjustment and other functions. A dual-input single-output fuzzy controller was designed and simulated. Simulation results showed that the controller could control the amount of spraying with the speed and density in real-time changed. An indoor test bed which could simulate cotton environment was built and the spraying system was tested. The results showed that over 90% weeds were sprayed with speed of 0.2m/s.