Abstract: Light propagation characteristic of dual defect microcavity inserted or replaced by photonic crystal were studied by numerical calculation and simulation. The result shows that when the periodicity of dual defect becomes larger, there will be more defect modes in microcavity transmission spectrum, the modes in replaced dual defect microcavity are more than that in inserted dual defect microcavity. As the high-refractive medium of defect thickens, the defect modes in dual microcavity draw close to the center and tend to degenerate; at the same time, pairs of double defect modes appear on both sides of band gap, with more on the high frequency side and less on the low frequency side; however, the defect mode quantity in replaced dual defect microcavity is larger than that in inserted dual defect microcavity. While the low-refractive medium of defect thickens, defect modes of inserted dual defect microcavity move to low frequency and tend to separate for function coupling, with the transmission index decreasing; but the defect modes of replaced dual defect microcavity draw close to the center and tend to degenerate, and meanwhile the whole defect modes slowly shift to high frequency and the transmissivity remains 100%. Theses light propagation characteristic provide design reference for optical filter, switch and laser devices.