A new methodology was presented for automatic detection of rock mass discontinuity trace.Based on edge detection and filter of characteristic,using of the differences of specific mathematical features on the connected pixel region between discontinuity information and non-discontinuity noise,non-discontinuity noise was filtered out and discontinuity information was left by this new methodology.Then,the discontinuity traces were detected automatically.Taking the image of rock mass discontinuity in Beishan for example,the image was processed in MATLAB by this methodology.The results showed that for a large variety of complex discontinuity image,the methodology can quickly and accurately detect the trace of rock mass discontinuity automatically.