As the basis of application of one-dimensional photonic crystals, the investigation of photonic band gap is very important. The properties of one-dimensional photonic crystal′s band gap and the factors affecting it are investigated through transfer matrix method. The importance of relative bandwidth to the design of photonic crystal is described. Based on these discussions, the methods used to extend the band gap of 1-D photonic crystal are discussed. A new method, using some 1-D photonic crystals to extend the band gap at angle space, is proposed. By using this method, some structures composed of 2, 3 or 4 1-D photonic crystals are designed. At the end, the reflection of two structures at optical frequencies are calculated. The relative band width of hybrid structure consisted of four 1-D photonic crystals is up to 57.52%, which strongly indicates that the extension of relative band width of 1-D photonic crystals based on incident angle domain is very effective.