The invention relates to a device and a method for monitoring irradiation field on line in real time. The device for monitoring an electron beam irradiation field on line in real time comprises an irradiation field detection device, an analysis device and a control device, wherein the irradiation field detection device is used for receiving electron beams at an edge part and is placed on a position where the electron beams at the edge part of the electron beam irradiation field can be received and the normal scanning range of the electron beams is not blocked; the analysis device is connectedwith the irradiation field detection device and used for analyzing and processing signals of the electron beams, which are received from the irradiation field detection device; the control device is connected with the analysis device and used for receiving an analysis result signal and outputs the distribution information of the irradiation field according to the received analysis result signal. The device has the advantages of simple structure, low production and manufacturing costs and real-time detection to the distribution of the irradiation field, thereby improving the irradiation processing process and the processing quality.