The influences of the remote gain unit (RGU) location on ultralong span repeaterless optical transmission system with different bit rates and the number of the signal channels is discussed in this paper. The experimental results show that the optimal location of the RGU can always exist for obtaining the minimum signal bit error rate at receiver with a fixed transmission distance. The optimization location of the RGU is entirely dependent on the total transmission length and the number of channel in the wavelength division multiplexing (WDM) system. When the 1480-nm pump power is set about 1 W, the span of the repeaterless WDM transmission system can be extended to 402 km through optimizing the location of the RGU. The optimal configuration scheme of the RGU can provide a potential application to increase the transmission spans for undersea transmission systems, electric power transmission systems, and the other special trades.