Based on the coupled-mode theory and transfer matrix method, the ultra-wideband filtering characteristics of chirped long-period fiber gratings (LPFG) are analyzed. When the resonant peaks corresponding to different order cladding modes overlap with each other, the transmission spectral bandwidth of chirped LPFG can expand to 500 nm, so this chirped LPFG can be used as ultra-wideband band-stop filters. With the increasing of mode order, chirp coefficient, grating length, grating period and refractive index modulation, the transmission spectral bandwidth increases monotonically, but the transmission spectral bandwidth decreases with the increasing of the fiber cladding radius. The refractive index apodization is proposed to inhibit the side-lobes of the transmission spectra. It provides a new method for the design of wideband filters.