Dichroic beam combiner is the kernel technology of the dual mode guiding simulation system. Based on the photonic band gap structure of one-dimensional Photonic crystals, a new method of designing a diachronic beam combiner is proposed in this paper, through which mid-IR region high reflection mirror coating is designed and calculated by using plane-wave expansion method. Simple construction, combination of broad wave band beams in 2D and wide-angle is realized, and polarization of off-axis incident beams is prevented. The analysis of infrared reflectivity and radio frequency transmission rate demonstrates that this new method can perfectly satisfy the demand of design.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.