Abstract: In order to improve the inversion efficiency,a method for quickly and directly estimating the sound speed of the surface layer of the seabed is proposed.According to the energy flux theory,the ocean ambient noise data received by the vertical line array can be used to passively extract the bottom loss.Obviously,the bottom loss curve has an effect of critical angle,which is used to estimate the sound speed of the surface layer of the seabed.Based on the ray model,the difference between the bottom loss of noise extraction and the theoretical value is derived,while the beamforming performance of the array at different angles and frequencies is discussed.Considering that the sound ray will bend under the environment of non-constant sound gradient,the angle needs to be corrected to improve the universality of the method.There is a corresponding relationship between the critical angles of different frequencies and the effective depth.Data processing result of the Yellow Sea shows that within the effective depth where the critical angle remains unchanged,the surface layer of the seabed can be regarded as a constant speed layer.The sound speed of the surface layer of the seabed in this sea area is estimated at 1547 m/s within 0.5 m,which is similar to the active inversion result.