Abstract: A semi-automatic detection method of rock mass discontinuity trace based on image processing was presented.The basic ideas are as following:A small number of main crack control points which are difficult to identify are manually designated in the image,and then the image is divided into two parts,and edge detection is carried out respectively.Two segmented line pictures including discontinuity trace and fake edge are obtained.According to the principle of distance and angle,the segmented lines in the two images are connected iteratively.Finally,the results are superposed to get the complete fissure trace detection results.This method could reduce the interference between the primary and secondary fissures by detecting them separately,and ensure the integrity and correctness of the identification of the primary and secondary fissures with higher accuracy.