Based on the coupled-mode theory,the transfer-matrix method is used to calculate the transmission spectra of π phase-shifted long-period fiber gratings(LPFG). It is found that multi-π-phase-shifted LPFG is a filter with two stopbands and one passband,and the pass band width increases with number of π phase shift. Index apodization method and a new length apodization method are used to optimize the transmission spectra of phase-shifted LPFGs. And the easily realization of the new length apodization method can facilitate the fabrication of phase-shifted LPFG.