In coherent optical transmission system, the inaccurate bit synchronization induced by chromatic dispersion (CD) and local oscillation (LO)frequency modulation (FM) noise reduces the performance of the digital receiver system. To tolerate CD and time jitter, a modified Gardner phase detector for coherent optical differential quadrature phase keying (DQPSK) signal is presented. The proposed scheme, utilizing the features of the signal and inter-symbol interference (ISI), also combining the information both time and frequency domain of the signal, relieves the impairments to the timing recovery caused by CD and noise. A 40 Gb/s polarization division multiplexed DQPSK (PM-DQPSK) signal transmission and coherent detection digital communication system is experimentally demonstrated. The result shows that the scheme is efficient in resisting CD and has a good performance on time jitter.