Abstract: The reflective characteristics of sea and sky background radiation,which have important effect on the infrared signature from flight vehicle above sea,are studied in the paper.The statistical distribution of wave facet directions is discussed,the effects of randomly riging and falling sea wave on the sea surface radiation and on the sun light reflection are also discussed.Calculation methods of irradiance from sea surface radiation and reflected sun light on observation point are derived in the paper.Finally a computation example is given,which shows the sea and sky background radiation has important effect on infrared characteristic especially in the front area of a flight vehicle.