Considering the disadvantages of internal contour imaging system, such as poor precision, a three-channel timing differences ranging method was put forward. It was achieved internal contour high precision imaging with FPGA and C8051F340 microcomputer as controllers. By the system, the echo time differences of three channels were obtained to calculate the testing distance through internal contour three-channel ultrasonic ranging, so the problem that internal contour of the measured objects were uneven surface was solved effectively, and the imaging accuracy was improved. The real-time rendering of the interior contours was achieved by MATLAB. The experiment shows the system has stable performance, high precision, large range, and will have a broad application prospect.