A structure and a digital signal processing algorithm of orthogonal frequency division multiplexing-passive optical network (OFDM-PON) downstream coherent detection system based on optical-comb wave are studied. A multi-wavelength optical-comb wave generator is used as the source of optical line terminal. One of the wavelengths is used as OFDM signal carrier and its adjacent wavelength is used as local oscillation light. In this way, the optical phase noise cancellation is achieved in coherent reception at optical network unit. Since the carrier offset and optical phase noise are avoided, the digital signal processing can be simplified. The comparison of the performance of three phase estimation algorithms, which are pilot aided (PA) algorithm, maximum likelihood (ML) algorithm and PA+ML algorithm, are carried out. It is found that the performance of the PA+ML algorithm is far more better. It can reduce the number of pilot subcarriers to 2 and the receiver sensitivity can be improved to -30 dBm. The hardware and software designs of the OFDM-PON system are expected to be a promise solution for the next generation optical access networks.