Abstract: The modulation effect of defect on the filter properties of one-dimensional photonic crystal was studied by transfer matrix theory and numerical simulation method. The result shows that when the defect unit is located in the center of the periodically arranged structure of photonic crystal, defect modes number and filtering quality are changed with the number of defect, but the transmittance remains the same; with the increasing deviation of defect unit from the photonic crystal center, the transmittance of each filter channel in photonic crystal will be lower, but the distance between the filter channels is unchanged; the filtering quality of each filter channel is improving with the increasing distance between two defect units, and the defect modes are degenerating; when defect thickness multiply increases, filtering quality of each channel is improving and multi-channel filtering appears. Effect of defect on filter properties of one-dimensional photonic crystal can provide theoretical guidance and modulation mechanism to design high quality optical filters and optical switches