Abstract: Provided is a loopback-type wavelength division multiplexing passive optical network (WDM-PON) system including: a central office converting downstream signals into downstream optical subcarrier multiplexed (SCM) signals to wavelength-multiplex and transmit the downstream SCM signals, or receiving and demodulating upstream optical on-off keying (OOK) signals to convert the upstream optical OOK signals into upstream signals; a remote node receiving the wavelength-multiplexed downstream optical SCM signals and wavelength-demultiplexing and transmitting the downstream optical SCM signals, or receiving the upstream optical OOK signals and wavelength-multiplexing and transmitting the upstream optical OOK signals; and subscriber interface units dividing the downstream optical SCM signals into first and second optical SCM signals, converting the first optical SCM signals into downstream signals, and modulating the upstream signals to the optical OOK signals by using the second optical SCM signals to transmit the opt