Abstract: The apical dominance of cotton was unfavorable to the grown of cotton. Cotton topping at the appropriate time can eliminate the apical dominance of cotton, achieving the purposes of precocity, high yield and stable the production. As such, topping based on artificial, topping based on chemical control method, and topping based on mechanical method are developed. However, there are some disadvantageous factors to restrict the promotion and application of topping based on artificial and chemical control methods. The cost of topping based on artificial method is too high, and is inadequate to apply in large scale. The stability of the topping based on chemical control method is low with some variable factors, and some environmental pollution problems are also an issue. Therefore, topping based on mechanical method is chosen as the suitable method to cotton topping. The low efficient is shown on current cotton topping machine, and the operations of topping and pesticide is achieved independently so that large energy and man power are wasted. Therefore, to overcome this disadvantages, in this study, an integrated idea of topping accurately and variable rate of pesticide spraying was presented, and a 3MDZ-4 self-propelled cotton topping and spraying machine was developed, with functions of the seedling divided, the seedling holding, the topping and the pesticide spraying simultaneously. The seedling divided-holding device, the height detection system, the topping system, and the spraying system were the critical components of the machine. To assure the quality of the designed machine, based on the design and analysis of the critical components, the critical parameters were obtained. The critical parameter of the seedling divided width was 470 mm. The critical parameter of the height detection range was 0 - 320 mm. The critical parameter of the minimum speed of disc cutter was 716 r/min. The critical parameter of the length of parallel four connecting rods was 640 mm. Through the field experiments, it was found that the topping efficiency was 91%, with the running speed of 2.5 km/h, indicated that the high efficiency of topping was realized. The coefficient of association of the cotton height was 0.81, indicating there was strong correlation between before topping and after topping. The spraying quantity was adjustable according to the running speed, the relative error of the actual flow and the theoretical flow was less than 5% so that the variable spraying could be achieved. The designed machine could realize the man-machine interaction. The current spraying quantity, the running speed, the working area and the spraying quantity of flow all could be display real-time on the touch-screen display. For the process monitoring, the sound and light alarm was achieved with the pesticide liquid was lower than set value, which improved the intelligent level of the cotton topping and the spraying machine. This research could provide some theoretical references for design of the cotton topping and spraying machine.