Considering the sea water seepage, limit analysis by finite element method is used in a subsea tunnel to analyze the stability before and after grouting reinforcement. Results show that the faultrupture zone decreases the stability of tunnel. The wider the faultrupture zone is, the more unsafe the tunnel is. The more thickness the grouting loop is, the safer the tunnel is. And the faultrupture zone with slip surface of 45° tends to lose stability most. The width of subsea tunnel lining must increase, if the full seepage pressure is considered in the design of tunnel.