The main technical factors and structure of the 3WDZ-6 type cotton top cutter was introduced. The characteristics of the mechanical system were presented, the research on automatic and closed-loop control system, which adjusted according to cotton height were also done. The cotton height experiment was tested using laser and ultrasonic sensors; the experimental results showed that the measuring error rate of the laser sensor was smaller than that of the ultrasonic sensor. The control and mechanical systems would meet the cotton top cutting agricultural requirements. 