In order to solve the problems of poor imaging quality， complex structure and large volume of existing dual-band common aperture cameras， a visible/long-wave infrared dual-color optical system was proposed. Simultaneous imaging is achieved by sharing the front-end reflection structure in the two bands and adding a dichroic beam splitter on the back of the main mirror， thereby ensuring the compactness of the system structure. The influence of the dichroic beam splitter on the infrared band imaging was analyzed in detail， as well as the influence of different angles of the dichroic beam splitter from vertical direction. The correct system of long wave infrared band and the image plane were eccentrically processed by -2.39 mm， so the image quality of the infrared band was greatly improved. The validity of the analysis conclusion was verified by the field test imaging. The results show that the system has loose tolerances， simple structure， and a few optical components. This system has the advantages of easy processing， installation， and strong engineering feasibility， which can effectively improve the target detection and recognition capabilities of the camera.