Optical en/decoder is one of the key factors affecting the performance of optical code-division multiple-access (OCDMA) system. The mathematical model of OCDMA en/decoder based on phase-shifted superstructured fiber Bragg grating (SSFBG) with low reflectivity is presented, and the performance of SSFBG en/decoder is investigated with different input pulse width as well as grating chip length. Here, the auto-correlation ratios of PA to the maximum wing level (P/W) and cross-correlation level (P/C) are used to quantitatively evaluate the en/decoder performance, and the curves of the relationship between P/W, P/C and pulse width as well as the grating chip length are obtained. From the analysis of simulation result, we can see when the pulse width and grating chip length meet some condition, optimum en/decode performance can be obtained, namely, an appropriate grating chip length en/decoder can be selected accroding to input pulse width, which can result in a better system performance.