Based on the wideband detection model, the accuracy and the volumes of spectral data combined with the narrowband information are applied in targets recognition, and then a new method to select the narrow detection waveband of infrared warning satellites based on synthetic signal-to-noise ratio is proposed. According to the narrowband imaging principle of warning satellites, a narrowband filter is added in the imaging system. The radiance from target and background, the dark current noise as well as the thermal noise from detector are considered, and then a new complete model for calculating synthetic signal-to-noise ratio is proposed. Taking the defense support program（DSP） satellite as an example, the central waveband of 2.726, 2.835, 3.012, 4.339 μm with the detection bandwidth of 10 nm, as detection waveband, is selected to calculate the synthetic signal-to-noise ratio spectra of tail at various altitudes.