Modulation principle of dual-polarization quadrature phase shift keying (DP-QPSK) signal and polarization-switched quadrature phase shift keying (PS-QPSK) signal are demonstrated and advantages of PS-QPSK are proved in theoretically. A novel modulation scheme of PS-return-to-zero(RZ)-QPSK signal is proposed. The scheme reduces transmitter cost by less use of Mach-Zehnder modulator (MZM), but also presents similar performance as traditional structure for PS-RZ-QPSK. The simulation results indicate PS-RZ-QPSK can achieve a better transmission performance than DP-RZ-QPSK at the same bit rate (84 Gb/s) and baud rate (28 GBd), and prove the feasibility of novel modulation scheme.