According to the characteristics of anti-explosion analysis about the small cylindrical shell structure under water, in which the wave scattering is so great that it cannot be ignored, an approximate procedure is proposed to analyze the shock load of the structure suffered underwater explosion. The shock wave is decomposed as the summation of plane harmonic waves with various frequencies, then the scattered acoustic field around the cylindrical structure is studied, by which the overall pressure distribution on the surface of the structure can be obtained. The average overall pressure of the structure suffered underwater explosion can be calculated also, which can be considered as the underwater explosive load. An underwater explosion trial is done in a lake to validate the proposed procedure, and the results show that the overall pressure distribution on the surface predicated by the procedure theoretically nicely agree with the results of trial. Then, the time history of stresses from the trial is quite close to that from numeric simulation. The procedure is understood easily, and can be applied in analyzing the time history of stresses of small cylindrical structures impacted by shock wave, in which the wave scattering of structures plays a key part.