The invention relates to a marine reactor and once-through steam generator secondary side passive residual heat removal system, comprising: an once-through steam generator arranged in a containment and provided with a first inlet and a first outlet; a water supplement box that stores a coolant and is arranged outside the containment, wherein the water supplement box is provided with a second outlet and a second inlet, and the second outlet is positioned below the liquid level in the water supplement box; an incoming pipe connected between the first inlet and the second outlet; an outgoing pipeconnected between the first outlet and the second inlet. The liquid level in the water supplement box is higher than the first inlet, the coolant in the water supplement box is allowed to flow into the once-through steam generator via the second outlet and the first inlet, the once-through steam generator adsorbs atomized steam that flows out of the first outlet, condensate flows into the water supplement box via the second inlet. The system of the invention employs the passive residual heat removal technique, and the removal of residual waste when the reactor loses a normal heat removal pathis ensured; the water supplement box is communicated with the once-through steam generator, so that automatic water supplementing is achieved.