In modern dynamics system, the radiant temperature of the flame, which caused by the transient plasma stimulated by high-energy-level electromagnetism field, takes an important role in the description of the flying object’s status as well as cauterization of the trajectory. Due to its extremely high temperature and transient process, the radiant temperature of the flame can hardly be measured through contracted ways, either static ways such as traditional pyrophotometer or CCD arrays. In the present paper, the authors bring forward a novel pyrophotometer based on classical theory of Planck’s law (blackbody radiation law) and multi-channel spectrums radiation method. With this new type pyrophotometer, any spectrum can be selected out from the wavelength of 300 to 860 nm within 2 ns. Also, the application of high-definition diffraction grating and fibers can ensure the accuracy of selected spectrum. The results through a serial of experiments by using this theory as well as high-speed photodetector indicate that this method is valid and accurate for the measurement of the object’s surface’s radiant temperature.