Variation of the transient temperature of transient physical field can be diagnosticated indirectly by measuring the temperature of the plasma. Spectrum of the plasma is gathered by the telescope which is the prepositive optical device of the system. Four characteristic spectral signals from the atom excitation are separated accurately (Δλ<0.1 nm) from the light of the plasma by the diffraction grating. Photomultiplier tubes (PMTs) with high sensitivity and short response time (＜1 μs) detect the four signals through the optical fiber, for satisfying the request of the transient measurement. Calculation precision （better than 2%） of the excitation temperature is increased by fitting Boltzmann line with four data. Computational model of the radiation temperature is deduced based on Planck’s blackbody radiation theorem. Transient excitation and radiation temperatures of the plasma are calculated isochronously by measuring light intensity once. Modulus of the spectral response of the system is calibrated by standard lamp. The results of the experiment show that the precision of the system is better than 3%.