Abstract: The radiant temperature measurement of the flame plays an important role in steel smelt and welding or dynamic launch system for the description of steel components and flying object 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 by traditional methods. A novel pyrophotometer based on classical theory of Planck-s law (Blackbody radiation law) and multi-spectral radiation method was put forward. With this pyrophotometer, any spectrum can be selected from the wavelength of 300 nm to 860 nm within 2 ns. The use of hig-resolution diffraction grating and fibers ensures the accuracy of the selected spectrum. This instrument provides accurate radiant temperature measurements of object surface and real-time spectral emissivity. The experiments of high-speed flying object and bromine tungsten bulb were conducted. Experimental results indicate that this method is feasible and accurate.