In order to mitigate the problem of strong nonlinear effect in the long-distance unrepeatered transmission system, the performance of phase conjugated signals was studied. The performance difference between single polarization and phase conjugated system was analyzed by simulations. Simulation results show that the dispersion pre-compensation of the phase conjugate signals is different from that of the relay transmission system. Compared with the single polarization signal, when the transmission distance is 330km and the backward pump is set to be 0.8W, the BER performance of the conjugated signal can be improved by more than 2dB for 32-GBaud QPSK when the forward pump is 0.4, 0.6 and 0.8W, respectively. Meanwhile, the transmission distance of the phase conjugated signal is increased by about 10%.