In view of the current situation of the height adjustment of combined harvester relying on the experience of the operator, a kind of the adaptive height adjustment system is designed for the Bilang 4LZ–1.2 crawler self–propelled combine–harvester. The system is mainly composed of the crop height detector, the header height detector, the hydraulic actuator and the control unit. The height of the crops and the header were detected by the unilateral infrared reflection device and the lifting height of the hydraulic cylinder with displacement sensor, respectively. The rising and falling of the header was implemented by changing the closing time and the direction of the electromagnetic valve which was controlled by using fuzzy PID algorithm and taking the crop actual cutting height as control objective. The test results showed that the maximum error of the height adjustment is 2 cm, error range of crop height detector is 0 to 2 cm, and the response speed of header in rising and falling are 0.22 m/s and 0.16 m/s, respectively.