The invention relates to a sodium-cooled fast neutron nuclear reactor system. The sodium-cooled fast neutron nuclear reactor system comprises a pressure vessel, a core arranged in the pressure vessel,a loop driving unit and an intermediate heat exchange unit, and a second loop heat exchange unit and a second loop drive unit disposed outside the pressure vessel. The intermediate heat exchange unitcomprises a primary side channel and a secondary side channel; the primary side channel and the primary loop driving unit, and the core constitute a primary side loop, and sodium circularly flows along the primary side loop, the primary loop driving unit, and the core. The second loop heat exchange unit includes a hot channel and a cold channel; the hot channel, the second loop driving unit and the secondary side channel form a secondary side loop, so that sodium circularly flows along the hot channel, the second loop driving unit and the secondary side channel, and the carbon dioxide is circulated in the cold channel. A coolant sodium has excellent heat transfer properties, improves cooling efficiency and thermal efficiency of a reactor. The sodium flow containing a radioactive substancein the primary side loop is isolated from the outside, and radioactive massive leakage due to leakage of the coolant sodium can be eliminated.