The invention relates to a section milling type casing pipe sidetracking windowing tool, and belongs to the technical field of petroleum and gas well casing sidetracking windowing tools. The section milling type casing pipe sidetracking windowing tool is composed of an upper connector, a tool body, a flow guide cone, a flow guide cone seat, a flow guide rod, a piston assembly and the like, and is characterized in that the upper connector is arranged at the upper end of the tool body, the flow guide cone seat is arranged on the upper portion of a cavity of the tool body, and the flow guide rod and the flow guide cone are in clamped connection; a small spring is arranged at the connecting position of the flow guide cone and the flow guide rod, and the piston assembly is arranged at the position, on the lower portion of the flow guide cone seat, in the cavity of the tool body, and is composed of an upper piston body, a throttle pipe, a lower piston body, a spring, a limiting pressing plate, a supporting sleeve and a supporting nut; a knife rest is arranged on the lower portion of the tool body, and provided with knives through pin shafts; the lower piston body is provided with the supporting sleeve through the supporting nut, and the limiting pressing plate is arranged below the lower piston body. The stretching amount of the knives can be adjusted and controlled, and the knives can be automatically opened or contracted, and are solid and durable. Operation is convenient, the sidetracking windowing effect is good, and milling efficiency is high.