With the continuous improvement of single carrier transmission rate, it is great challenging to increase the transmission distance in unrepeatered link. This paper demonstrates a unrepeatered transmission of single-carrier 400 Gbit/s over 501.18 km with 78.68 dB span loss using single fiber for the first time. The technology of high-order pumps, forward and backward remote optically pumped amplifier, sub-carrier frequency-division multiplexing and ultra-low & large effective area fiber are used in this experiment. The transmission wavelength, the power of the high-order pump and the location of the remote gain unit are also optimized in this work.