Paver can finish paving, ramming and pressing in a time, with certain density, thickness, width. It is suitable for various grades of highway, airport and road construction. The auto-levelling system, as an important part of paver, makes paver simple operation, safe and reliable, greatly reduce the loss of raw material and manpower, and the control accuracy directly influences the quality of paving flatness. The problems faced of auto-levelling system are introduced firstly. Secondly, ultrasonic sensor range finder technology at home and abroad and the present situation and basic principle, characteristics of ultrasonic sensor are clarified. Third,the range affecting factors of the ranging accuracy are found and appropriate method is put forward to compensating and correcting. Eventually, the method applied in the CPLD EPM570T100C5N, based on the C8051F040 singlechip , ultrasonic ranging circuit are formed. This paper illustrates the hardware and software design of every circuit module.