Using the optical coherent superposition principle, the transmittance formula of band is derived for one-dimensional finite-period photonic crystal, and the coherent superposition theory is established. The coherent superposition theory and the characteristic matrix theory are compared and the simulated results are consistent. The coherent superposition theory has advantages of the characteristic matrix theory, and overcomes the lack of the characteristic matrix theory. It is a better way to study band of one-dimensional finite number cycles photonic crystal.