The omnidirectional reflection properties of one-dimensional photonic crystals are studied by using the bandgap theory deduced by the transfer matrix method. Investigations show that the constructure parameters affect the omnidirectional reflectivity band of a photonic crystal significantly. In order to increase the omnidirectional reflectivity band, some methods are proposed to increase the refractive index of dielectrics, the ratio of the high and low refractive index, and make the ratio of the optical thickness near to 0.85. The conclusion is helpful for designing a dielectric omnidirectional reflector.