A signal modulation format which is suitable for intensity modulation dire ct detection (IM/DD) optical system is proposed.The signal based on Nyquist pul se subcarrier modulation (SCM) can effectively improve the spectrum efficiency.In order to reduce the performance deterioration caused by the system transfer function,digital pre- and post-equalization are applied in the system.We establish the experiment setup of polarization division multiplexing ( PMD) intensity modulation direct detection optical system with pre-equalization and analyze the peak to average power ratio (PAPR) of Nyquist pulse subcarrier modulation signal.The experiment successfully achieves the bit error rate (BER) of 64QAM Ny quist pulse subcarrier modulation signal less than forward-error-correction (FEC) threshol d after transmission along 20km standard single-mode fiber (SSMF)for the firs t time.