In order to quantitatively analyse the polyphase contents of metallographic structures, this paper presented an enclosing image edge extraction algorithm based on the Canny algorithm and a grayscale contour line.Firstly, the grayscale images of metallographic structure were blured with Gaussian blur,and then the initial edges with single edge effect in blured images were acquired using Canny algorithm. The thresholds were automatically calculated by the maximising variance between-class to reduce false edges and to obtain the basic edge for the next steps. According to the grayscale neighborhood of the end points of basic edge, the gray values of grayscale contour line were calculated. On the basis fusion conditions of basic edge and grayscale contour line, the closing edge could be created from the end points of basic edge. Experimental results indicate that this algorithm can generate an enclosing edge efficiently. The error rate of the polyphase contents of metallographic structures has decreased to ±1% by using this algorithm. The proportion range of different sections about one metallographic image has increased to 5%-95%. The algorithm can satisfy the demand of quantitative analysis of the polyphase structure contents and is suitable for different shape metallographic structures.