For investigating influence of structure scale on dynamic characteristics of water jet, numerical simulation of underwater explosion near three sizes of spherical wall was carried out. The result shows that the smaller scale of the wall is, the sooner jet penetrates bubble, the smaller and slenderer bubble is, the higher speed of jet is, the smaller width of jet is, the weaker jet impacts the wall, the smaller impulse is. The result preliminarily reveals that the small scale structure can reduce water jet power, which provides a useful reference for warhead design for underwater weapons fighting against small targets.