The analysis theory of phase-shifted superstructured fiber Bragg grating (SSFBG) encoder/decoder is detailed, in the basis of transfer matrix method (TMM), and the design methodology of such the encoder/decoder is developed, which is validated by the simulation results of the 127-chip 2-phase and 4-phase encode/decoder. The design methodology and results possesses guidance as well as reference value for the analysis of phase-shifted SSFBG. Meanwhile, the sensitivity difference between 4-phase family A code and 2-phase Gold code are also discussed, and the results reveal that the former codes require much higher precision of fabrication technology of gratings, than the latter ones under the scenario of the same code length.