The influence of background radiation on noise of medium-wave and extremely-long-wave HgCdTe detectors is studied. It is found that the noise of extremely-long-wave detector decreases with the background radiation increasing, while for the medium-wave detector is shows the different way. Noise spectra show that the main noise sources are generation-recombination (G-R) noise and 1/f noise which have the similar trend while the background is changing. The G-R noise for different wavelength response detectors is calculated using carriers and effective lifetime theory. There exists a maximum value when G-R noise changes with background radiation. While the noise is on the different position for medium-wave and extremely-long-wave detectors, the influence of background radiation on their effective lifetime is different. So noise behaves differently. Based on these, a new concept of “critical-background-radiation-flux-density” is put forward.