Scan efficiency η of infrared imaging system is related with the noise equivalent temperature difference (NETD) of the system.Another kind of expression of η can be derived from NETD theoretical formula and test data of detector such as relative spectral response,blackbody detectivity and et al,in which the spectral integration can be solved easily by doing numerical integration with spline function commands provided in MATLAB.This expression may be used as an indirect testing method of scan efficiency η.