The invention relates to the technical field of book binding, and in particular to a novel book cutting and positioning rule mechanism in a three-side trimming machine. There is a servo power transmission device fixed on the frame, and the servo power transmission is connected with a first moving frame through a gear and a rack, and the first moving frame is provided with a first guide shaft, a control motor and a second moving frame , the control motor drives a screw through a nut, and the screw is movably matched with the second mobile frame, and a second guide shaft is movably pierced on the second mobile frame, and the end of the second guide shaft is connected with the fixed piece Fixing, the positioning block is arranged on the said fixing piece, and its position of the positioning block between the two cutting positioning rules is relative. The invention has the advantages of simple structure, short adjustment time and improved positioning accuracy.