The invention discloses an automatic speed control system for an impurity removing fan in a sugarcane combine harvester. The system comprises pressure sensors mounted on a walking motor, a cutter headmotor, a cut-off motor and a fan motor, and speed sensors mounted on the walking motor and the fan motor. The output ends of the four pressure sensors and the two speed sensors are connected with theinput end of a control unit, the output end of the control unit is connected with the input end of a proportional amplifier, and the output end of the proportional amplifier is electrically connectedwith a proportional flow valve for controlling the speed of the fan motor. The control system can identify three different working states of the sugarcane combine harvester, match a target speed value of the fan according to the working state, and make the fan speed reach the target speed value by adjusting the opening degree of a valve port of the proportional flow valve, so that the impurity content rate and the loss rate of sugarcane in the sugarcane harvester are reduced, the whole control process has high automation, high accuracy, high stability, simple structure, and the control systemhas practical application value.