Abstract: The first-order small slope approximation is applied to the problem of the sound transmission from an airborne source into shallow water through a rough sea surface.By assuming a three-layer homogeneous fluid model and decomposing the sound field in water layer into up-going and down-going waves,the expressions for the down-going wave and thus the total field are derived.The expressions can be calculated by the FFT,but the alias problem will arise.To mitigate the alias effect,additional medium absorption is introduced and afterwards compensated.Monte Carlo calculations are performed.The sea surface is assumed to be random rough with a Pierson-Moskowitz spectrum and to have height variations in only one dimension.The results show that the mean field intensity decreases as the root-mean-square surface height increases,while the mean-square field intensity increases slightly.