The invention relates to a side hole machining jig which comprises a tool apron, a tool clamp and a locating assembly. An input shaft used for being in matched connection with a machine tool spindle is arranged at one end of the tool clamp, a chuck in transmission connection with the input shaft is arranged at the other end of the tool clamp, and the chuck is used for clamping a tool handle of a machining tool. The locating assembly comprises a support and a sliding block, one end of the support is fixedly connected to one side of the tool apron, and the sliding block is in sliding fit to theother end of the support. The tool handle is provided with two oppositely-arranged locating faces, a guide groove communicating with the end face of the sliding block is formed in one end of the sliding block, and the locating faces are used for cooperating with the two opposite side walls of the guide groove so as to promote the lengthwise axis of a blade to be parallel to the lengthwise axis ofa side hole of a to-be-machined product. The machining tool is located through the locating faces and the guide groove, and the lengthwise axis of the blade is parallel to the lengthwise axis of the side hole of the to-be-machined product; locating is achieved through the machine tool spindle, tool descending action is executed, and the blade can reach the lower tool position; and the structure issimple, use is convenient, and the machining efficiency is high. The invention further provides a side hole machining system and machining method.