Abstract: Measurements of lake-bottom reflectivity for grazing angles between 7 and 90 were made using a direct measurement in a lake. Acoustic and physical properties of the cores collected in situ were analysed in detail. It is proved that the bottom of the lake to be a low-velocity sediment. A theoretical model of four-layer fluid was built according to the cores, the layers were assumed to be homogeneous and absorbing. Theoretical computations were made using the model, comparisons of theoretical and measured reflection coefficients were made at 500, 1,000, 6,300 and 10,000 Hz, and a relatively satisfactory result is obtained. Some discussion is given in the last part of the paper.