Abstract: There are problems in gas monitoring using portable gas alarm in working face of coal mine,such as gas concentration reporting is tedious and boring. In the paper,camera was used to monitor portable gas alarm real-timely and methods of image processing and automatic identification of images collected by the camera were also put forward. The specific processes in automatic identification of gas concentration including image preprocessing,detection of image edge,and segmentation of displaying region of gas concentration were introduced in details,namely using image graycale method and mid-value filtering method to preprocess the monitored images,using Roberts operator,Prewitt operator and Sobel operator to extract image edges separately and using Hough transformation to detect straight lines of the edge, so as to segment target images.