Considering the factors such as the size of the target,the level of detection,detection probability and atmospheric environment,minimum resolvable temperature difference(MRTD)of thermal imaging systems is studied.An improved MRTD model is established.As atmospheric transmittance is related to wavelength and distance,atmospheric transmittance is calculated by spectral bisection method.Detection wavelength range can be divided into a series of equal narrow bands.The atmospheric transmittance of each narrow band is calculated by MODTRAN in a certain atmospheric condition and distance.Through the corresponding wavelength transmittance,MRTD of thermal imaging system is calculated.The accuracy and applicability of MRTD are improved by this method.