Differential transfer matrix method (DTMM) is an efficient tool to solve the wave propagation equation in inhomogeneous media analytically. Using it, the band structure of one-dimensional photonic crystals with continuous indexes of refraction is analyzed. The results show they also have clear band gaps. The more smoothly their refraction indexes change, the narrower their band gaps are. As to the photonic crystals with sine shape index of refraction, the more sharply their indexes of refraction change, the broader their band gaps and low their central frequencies are. The analysis method used is also applicable to one-dimensional periodical functional gradient materials.