The background radiation characteristic of space-borne environment is introduced.Taking laser warning system of LEO reconnaissance satellite with a height of 400 km for example,host satellite′s spectral irradiance model of every radiation source is built,and numerical calculation ia carried on;Supposing alarm wavelengh of laser warning system is 1.315 μm and the detetor′s working waveband is 1~3 μm,in the place of host satellite,the 1.315 μm spectral irradiance and the total irradiance of 1~3 μm of every radiation source is calculated.Through analysis and comparison,the results show that:the main background radiation sources of satellite borne laser warning system are the sun and the earth;the corresponding spectral irradiance in the alarm wavelengh is 39.9 mW/cm2 and 10.9 mW/cm2,and in the waveband of 1~3 μm,the total irradiance is 38.9 mW/cm2 and 10.35 mW/cm2.So effective background suppression methods must be studied to eliminate the influence of solar radiation and terrestrial radiation.