Taking the output signal to noise ratio ( SNR ) of systems as a criterion, the relationships among the detecting distance R , the noise equivalent temperature difference NETD , the minimal resolvable temperature difference MRTD , the minimal detectable temperature difference MDTD and the modulation transfer function of IR scanning detecting systems ( SDS ) were established. The testing methods for these figures of merit were also studied.As a result, the bases of the synthetic performance evaluation and the practical testing methods of IR SDS were provided.