Based on the theoretical analysis and data simulation of the characteristics of the return signal, the factors influencing the capability of minimum depth of an airborne laser bathymetry and the improved technical methods are discussed. The use of narrow laser pulse, high-speed detector and small field-of-view can decrease the overlap of the surface and the bottom reflected signals. Based on the analysis of the characters of the surface and bottom reflected pulse, a method using double-Gaussian pulses fitting to separate the surface and the bottom reflected signals from the overlapped returns and to improve the capability of minimum depth using an airborne laser bathymetry is proposed to meet the applications of coast measurement. A simulated experiment was conducted and a minimum depth of 0.4 m has been achieved with the proposed method. The measurement results of the avalanche photodiode (APD) and the photomultiplier tube (PMT) are compared, the similar accuracy was obtained.