With the development of infrared technology,high performance infrared microscope system plays an important role in scientific and technological fields such as thermophysical chemistry,microbiology and MEMS design optimization. In this paper,the structure and design method of an infinite image distance microscope system is proposed. An infinite image distance microscopy optical system is designed for MWIR detector with 320×256 array size,30μmpixel size and a relative aperture of 2. The designed system′s magnification is 3×,working distance is 35mm,and numerical aperture (NA) is 0.75. It can be applied to infrared microscopic detecting with high frame rate and similar microscope objective products.