Transmissivity of one-dimensional sine function photonic crystals is studied, and the characteristic of light transmission is analyzed. There are some important results: for one-dimensional sine function photonic crystals, the electric field can be increased or decreased at the defect layer, which depends on the refractive index of the defect layer; when the position of the defect layer is closer to the front side, the impact is larger on the distribution of electric field behind the defect layer. These results are important for the design and application of photonic crystals.