The invention discloses a passive residual heat removal system with a heat exchanger located at the exterior of a water tank for the secondary side of a marine pressurized water reactor, and relates to the field of nuclear reactor safety facilities. The passive residual heat removal system comprises a reactor pressure vessel, a containment and at least one heat removal subsystem, wherein the heatremoval subsystem comprises a steam generator, a condenser, a water tank, a main coolant pipeline, a steam pipeline, a condensate water pipeline and a cooling water circuit; and the steam generator comprises a steam generating part and a heat exchanging part. After an accident, residual heat in a reactor is discharged through the main coolant pipeline between the reactor pressure vessel and the heat exchanging part, the steam pipeline and the condensate water pipeline between the steam generating part and the condenser, and the cooling water circuit between the condenser and the water tank. According to the invention, by utilization of device position difference and medium density difference, a heat transfer medium in the system provided by the invention can passively flow, so the purposeof safe and passive residual heat removal is achieved.