For overcoming the shortcoming of large thickness homogeneous target, research on metal jet penetration into aqueous medium spaced target. The process of metal jet penetrating into aqueous medium spaced target was numerical simulated with AUTODYN. The interaction process among metal jet and air model, water model, spaced target model was established. On this base, velocity decay principle of metal jet penetrating into aqueous medium spaced target was investigated and compared with air medium spaced target. The results show that metal jet penetrating into aqueous medium spaced target can effectively decay the metal jet, and provide the basis for designing the aqueous medium spaced target.