The invention discloses a cutting temperature monitoring device of side milling process. The device comprises a temperature module support component and a temperature monitoring system, wherein the temperature monitoring system is mounted on the temperature module support component; the temperature module support component is mounted on a main shaft of a vertical milling machine and adjusts the axial and radial positions of the temperature monitoring system along the main shaft; the temperature monitoring system measures temperatures at different angular positions of a cutter surface in an empty cutting area in a synchronous and online manner, and performs fitting processing on the measured temperature data, so that a temperature range and a temperature distribution condition in the cutting area are obtained. By the cutting temperature monitoring device of side milling process disclosed by the invention, non-contact type measurement is realized; the temperature condition in the cutting area is obtained through processing the measured temperature data of a plurality of points on a cutter; the measurement of a three-dimensional temperature field in the cutting area is completed; and the device has the advantages of convenience in operation, accuracy in measurement, and comprehensiveness in data.