Abstract: One kind of the SAW seam tracking system with contactless ultrasonic sensor is presened.The working principle of a new ulrasonic sensor is explained.The Fuzzy-P control theory with Self-modulation scaling factor is applied in the seam tracking system.Based on the theoretical analysis and experiments,the hardware and software of the seam tracking Fuzzy-P controller are developed,which may significantly increase system's respones speed and tracking precisition.