First, the spectral characteristics of multi-phase-shifted fiber Bragg gratings (MPSFBGs) were studied theoretically. The results confirmed that we could acquire the spectra with narrowband flat-top filter response by accurately designing the key parameters of the MPSFBGs, such as the number and position distribution of phase shifts, the coupling coefficient and length of the gratings. However, the process errors introduced in the inscription process would worsen the filter spectra. To this end, MPSFBGs were inscribed by a post-ultraviolet-radiation technique and the relevant spectra were measured. The results demonstrate that the experimental spectra are in agreement with the ideal simulation results when the MPSFBGs have a large bandwidth, indicating that the control accuracy can meet the requirements of the gratings. However, the performance of the narrow-bandwidth filter with a larger coupling coefficient is more sensitive to the process errors. Furthermore, the process errors were analyzed and discussed. It turns out that increasing the length of MPSFBGs can effectively widen the tolerance of phase-shift position distribution, and phase-shift difference, and weaken the effect of UV-induced loss on the loss characteristic of the filters.