The invention relates to a single-phase moderator natural circulating device applied to a reactor cold neutron source, comprising a shell-pipe type heat exchanger; a pipe-side inlet of the heat exchanger is communicated with an outlet of an inner flow channel in an ascending tube and a connecting tube of a moderator storing tank, and a pipe-side outlet thereof is communicated with an inlet of an inner flow channel of a descending tube; a shell-side inlet of the heat exchanger is communicated with a cold source air supply tube and a shell-side outlet thereof is communicated with an inlet of anouter interlayer of the descending tube; an outlet of the inner flow channel of the descending tube is communicated with an inlet of an inner flow channel of a cold bag, an outlet of the outer interlayer of the descending tube is communicated with an inlet of an outer interlayer of the cold bag; an outlet of the inner flow channel of the cold bag is communicated with an inlet of the inner flow channel of the ascending tube, an outlet of the outer interlayer of the cold bag is communicated with an outer interlayer of the ascending tube, and the lower part of the outer interlayer of the ascending tube is communicated with a cold source air return pipe; and the pipe side of the heat exchanger forms a moderator circulating loop with the descending tube, the cold bag and the inner flow channelof the ascending tube, and the cold source air supply tube, the shell side of the heat exchanger, the outer interlayer of the descending tube, the outer interlayer of the cold bag, the outer interlayer of the ascending layer and the cold source air return tube form a refrigerant loop.