Abstract: In order to detect the process of molding plastic pipe wall thickness and diameter of the ellipticity change, a plastic pipe forming ultrasonic detection system online was developed. The thickness measurement theory and working mechanism of the system were analyzed, and the detected data processing method was focused. For the specific data processing requirements, a CIP-51 system SOC chip C8051F121 was used, its built-in programmable counter / timer array(PCA) could achieve high-precision time width of square wave digital sampling, providing the hardware basement for ultrasonic on-line monitoring.