"Grass snake grey line" is a method used for plot structure, and its metaphorical meaning is: like a snake in the grass, the line in the gray seems to be broken and continuous, and the shape is also broken and continuous. The article is intermittent and ups and downs. In writing, we skillfully handle the subtle relationship between characters and plots, and make use of various hidden and unreachable reactions that are not easy to be found to make some hints, just like the traces left by grass snakes and gray lines. In Water Margin, there are many examples of applying the method of "grass snake gray line". For example, when Song Wu came to Zishi Street, the author described the curtain for ten or twenty times, which naturally paved the way for the key plot that Pan Jinlian dropped the curtain and hit Ximen Qing on the head.