20-tone, 25 GHz spacing multi-carrier with the tone-to-noise ratio (TNR) larger than 25 dB is generated based on recirculating frequency shifter (RFS). Each subcarrier is modulated with a return-zero 16-quadrature amplitude modulation (RZ-16QAM) signal at 16 Gbaud and then polarization division multiplexing (PDM) is employed to further increase the capacity. At last, 2.56 Tbit/s, PDM-RZ-16QAM, coherent optical wavelength division multiplexing (CO-WDM) transmission over 792 km standard single-mode fiber (SSMF) with 25 GHz channel spacing using coherent detection is experimentally demonstrated. Compared to the theoretical limits, the measured optical signal to noise ratio (OSNR) sensitivity curve in back-to-back system is shifted by approximately 4.1 dB at a bit-error rate (BER) of 1×10-3. After transmission, the BER value of 4.5×10-3 at the optimal launchpower is under the threshold value of soft-decision forward error correction (FEC) of 2.4×10-2, and the corresponding constellations diagrams of x and y polarizations for recovered CO-WDM PDM-RZ-16QAM signals of the central channel show the excellent performance. The spectrum efficiency of our transmission system is 5.1 bit/(s·Hz).