The essence of safe return to port (SRtP) is that the passenger ships can withstand a fire or a flooding casualty. A flooding casualty, as a fire casualty, will seriously threat the safety of the passenger ships. The early detection, mid-term alarm and post-processing of the flooding casualty should be considered in the ship design. The redundant design of the flooding alarm system is studied for the 700 pax Ro-Ro passenger ship that was designed by Marine Design and Research Institute of China (MARIC). The relevant follow-up actions and processing measures of other SRtP subsystems are considered in view of the associated effects of the flooding casualty.