An apparatus for determining a downstream topology in a hybrid-fiber-coaxial network includes: a signal branching unit for branching a signal received from a cable modem termination system into a plurality of branched signals; a plurality of signal selecting units for selectively receiving a frequency band signal corresponding to a specific downstream channel among the plurality of branched signals; a downstream topology determining unit for analyzing a downstream ambiguity resolution frequency list contained in a channel information message of a downstream primary channel received from the cable modem termination system, collecting in parallel channel information messages with respect to a plurality of downstream channels, and determining a downstream topology at a high speed using the collected channel information messages; and a plurality of packet generating units for setting downstream channel frequency bands of the signal selecting units under control of the downstream topology determining unit, and sending the channel information messages of the collected downstream channels to the downstream topology determining unit.