Appling the numerical simulation method, the characteristics of underwater explosion loading in port were investigated. At first, the numerical method was proved by compared with the empirical formula and the results of experiments, then the propagation of shock wave of underwater explosion and the laws of bubble expansion were also analyzed. The result showed that: the loading of bubble expansion in an underwater explosion in port couldn’t be ignored. If the loading are scaled by impulse, the loading of bubble expansion is larger than the loading of shock wave in most area. The loading of bubble expansion and shock wave are influenced observably by the water surface, the closer to the water surface, the smaller the impulse of both. But the attenuation of the loading of bubble expansion is faster than shock wave, the impulse of bubble expansion even is less than the impulse of shock wave. Water bottom have less influence to the loading of bubble expansion, but have greater influence to the loading of shock wave. The acting time of shock wave would be shortened due to the influence of reflection of water bottom, thereby the impulse of shock wave reduced rapidly. On the condition of sludge bed in existence, the reflection of shock wave on the interface between sludge bed and water are not remarkable, most of the reflection wave is from the interface between sludge bed and rock.