The invention provides a device and method for measuring the output laser field shape of a terahertz quantum cascade laser. The device comprises a laser part and a laser detection part, wherein the laser part comprises a driving power supply, a cold head, a hot sink installed in the cold head, a terahertz quantum cascade laser installed on the hot sink and a polyethylene screen installed on the cold head; and the laser detection part comprises a thermal imager. The invention has the advantages that the thermal imager with mature manufacturing technologies, stable and reliable performance and real-time display function is used as a detection device of a laser beam output by the terahertz quantum cascade laser, the two-dimensional distribution situation of the laser beam section within a certain distance range away from the outgoing end face of the laser can be measured by the device in real time, and a good technical means is provided for the terahertz quantum cascade laser on performance testing in the imaging application aspect.