In order to guarantee out-of-gauge freight transport safety, it is necessary to study digital measurement to improve measuring accuracy for out-of-gauge goods which is very difficult to master outline dimension. The process of edge detection and the algorithm of canny operator edge detection were introduced in this paper. Taking the transformer for example, contour extraction was carried out on the end and the widths of transverse and longitudinal height are measured through the mathematical software MATLAB. Finally, measurement results are validated based on the known data. The results showed that canny operator has high calculate precision, and the closer to the part calibrated with pixel calibration, smaller the measurement errors are.