We propose a novel carrier phase estimation method for the high-speed coherent WDM PON based on the reflective semiconductor optical amplifier (RSOA). By using this method, we can utilize a cost-effective DFB laser as the seed light in such a network due to its fast trackability. We experimentally achieve an excellent receiver sensitivity of -30.8 dBm even when a DFB laser (linewidth: 3 MHz) is used as a seed light source in the 25.78-Gb/s upstream QPSK transmission over 20 km of SSMF.