The omni-directional transmission (reflection) properties were theoretically investigated for photonic heterostructures composed of cascaded one-dimensional photonic crystals. Each photonic crystal in the structure has the same materials and different optical thickness ratios of the alternate high-and low-refractive index layers. Taking two photonic crystals cascaded structure as the object of study, the change of photonic bandgap with different polarization and incident angle was investigated through transfer matrix method, the condition of designing omnidirectional reflectors with the maximum photonic bandgap width was obtained, i.e., the upper bandgap edge of the former photonic crystal overlaps with the lower bandgap edge of the latter photonic crystal at the maximum incident angle. The parameters of the omnidirectional photonic bandgap were given for the cascaded structure with the condition satisfied and not satisfied. The widths of the omnidirectional photonic bandgap were compared. The results show that the cascaded structure with the condition satisfied has the maximum omnidirectional photonic bandgap width.