To evaluate correlation characteristics for superstructured fiber Bragg grating (SSFBG) based en/decoder in the optical code division multiple access (OCDMA) system, the effects on encoding and decoding of input pulse width, encoder-decoder wavelength mismatch and grating refractive index modulation amplitude are considered. The theoretical results show that the degradation of en/decoder performance is obvious under the impact of increasing of input pulse width and encoder-decoder wavelength mismatch. In addition, there is a contradiction between correlation property and insertion loss of SSFBG en/decoder, it is need to select moderate refractive index modulation amplitude of SSFBG grating. The theoretical model of coherent time spreading phase coding OCDMA system based on SSFBG en/decoder is presented. Three different kinds of noise source are taken into account: multiple access interference (MAI) noise, beat noise and receiver noise. Moreover, the system performance degradation as a result of receiver’s bandwidth limitation is evaluated. Optical thresholder and turbo code are employed to improve the performance of coherent time spreading OCDMA system.