DP-QDQ is a partial-response shaping modulation based on 16QAM, which has been proposed in our previous research paper. In this paper, the power spectral density (PSD) of the DP-QDQ signal is analyzed and compared with DP-16QAM, DP-64QAM, and DP-256QAM signals, and the simulation setup is presented. The analysis results show the DP-QDQ signal has high spectral efficiency. In addition, the optical coherent DP-QDQ system is discussed in compared with DP-16QAM, DP-64QAM, and DP-256QAM systems in terms of the tolerance to optical signal to noise ratio (OSNR), transmission distance at 100G/s by simulations. It is revealed that DP-QDQ not only achieves high spectral efficiency, but also obtains a good compromise of the performances on spectral efficiency, OSNR tolerance and transmission distance.