A cost-effective ultra-dense wavelength-division-multiplexed passive optical network (UD-WDM PON) with speed of 12.5 Gbit/s and channel spacing of 12.5 GHz is proposed and demonstrated. The distributed feedback (DFB) lasers modulated in 4-level pulse amplitude modulation (4-PAM) format are used for downstream links, and the reflective semiconductor optical amplifiers (RSOAs) together with an optical frequency comb modulated in quadrature phase shift keying (QPSK) format are used for upstream links. We can achieve the error-free transmission of the upstream signals with speed of 12.5 Gbit/s even after 20 km single-mode fiber (SMF). The power penalty obtained by using the frequency comb generator instead of a tunable laser is around 0.5 dB. By using 11 DFB lasers and a set of intensity and phase modulators, it is possible to provide the seed light for 297 optical network units (ONUs) within the C-band.