Abstract: The infinite domain absorption boundary and explosion wave input method are two keys of numerical simulation research on underwater explosion. This paper draws on the seismic wave input method based on the internal substructure and the multiscale analysis method for explosion problems based on the substructure of explosion source and proposes a shock wave input method for underwater explosion based on the substructure of explosion source considering that the underwater explosion load and the seismic load belong to the same fluctuation problem. The method first decomposes the shock wave field in the explosion source region and the free wave field motions were transformed into the equivalent explosive loads, which enables the input of shock waves in the underwater explosion problem. In this paper, a circular substructure of explosion source is used, and a one-dimensional model in AUTODYN software is used to calculate the free shock wave pressure in this region under the action of the underwater explosion. Further, based on the continuous fractional approximation method, a high-precision time-domain artificial boundary condition is proposed to simulate the radiation effects of infinite domain, which can be placed close to the structure and explosive source substructure is proposed. The method proposed in this paper transforms the underwater explosion load through the substructure of explosion source and adopts the high accuracy absorption boundary to greatly reduce the calculation area, which not only ensures the calculation accuracy, but also reduces the number of elements, with high calculation efficiency and practicability. Finally the numerical example is analyzed to verify the accuracy of the model and method of this paper, The pressure time history curves of measuring points during shock wave and bubble pulsation stage under underwater explosion are simulated, and the effect of circular structure on the scattering effect of underwater near-field explosion wave is studied.