Polarized coherent light with high spectral efficiency advantage has been widely used in power optical fiber transmission. However, this transmission method has its sensitivity to outside interference. To reduce damage to the optical fiber transmission, and improve the SNR of channel, the efficiency modulation format is a effective way, improving the transmission of the channel properties and extending the transmission distance. By rotating the conventional QPSK constellation proposed polarization switching quadrature phase shift keying (PS-QPSK), at the receiving side with coherent detection technique can achieve high performance signal demodulation. For the fiber optic backbone transmission system using PS-QPSK, optical signal to noise ratio (OSNR) and transmission distance are improved. In the bit error rate of 3.8×10-3 channel SNR improvement of nearly 1dB, 50 GHz frequency fiber optic backbone maximum coverage increased by 30% compared to traditional methods.