As the total circumstance radiation which falls on the target being taken as the equivalent radiation of a blackbody at temperature T0, the effect of environment radiation change on the sensibility equation of thermal imaging system is studied, and consequently the ratio emissivity ε of the target can be determined theoretically through the change of environment radiation. This paper gives the method and the measurements of temperature of the environment equivalent blackbody T0 Also a new method is given about the thermometer measurement of the ratio emissivity of the target. When the environment radiation is not neglected, the problems are methodically and theoretically discussed about how to use a ratio radiometer to measure the real temperature T of the target and how to use the radiometer to measure the spectral reflectivity of visible light from ground target.