Most of the published literature relating to parallel optical path fusion focuses on analysis and research of image fusion algorithms; little literature discusses the optical system for image fusion. This paper designs and analyzes a fusion optical system of visible light /low-light and a long wave infrared image fusion and parallel light path layout according to the characteristics of a hand-held fusion observation lens. Firstly, the advantages and disadvantages of a shared optical path optical layout pattern and parallel optical path optical layout pattern were introduced. The optical layout pattern of the parallel optical path was selected according to the characteristics of the handheld fusion observation lens. Secondly, according to the requirements of the hand-held fused observation lens, the “heat dissipation difference” and fixed-focus optical design of the visible light /low-light optical objective lens were determined and the internal focusing optical design was performed for the long-wave infrared objective lens. Thirdly, the image registration accuracy was analyzed from the three aspects of the optical layout, magnification, and distortion of the parallel light path, according to the design of the fused optical system. Finally, the fusion image of the actual scene of the handheld fusion observation lens was analyzed and judged; it was found that the fusion image quality is suitable and the fusion performance of the handheld fusion observation lens can meet the requirements.