Based on the principle of external field splicing, a principle prototype of large-field mid-wave infrared was developed. A cooled mid-wave infrared optical system was designed. The secondary imaging structure of the optical exit aperture coincides with the cold screen was adopted to make sure 100% cold shielding efficiency of the system. The working wave band was 3.7-4.8 μm, the focus was 40 mm, the relative aperture was 1:2, the full field of view was 21.74°×17.46°(27.88°), the total length of the system was 145 mm. The principle of external field splicing with time-sharing exposure imaging of holes/mirrors was adopted, 2×1 field of view expansion was realized. The design result show that MTF>0.68 of the off-axis field of view at spatial frequency of 21 lp/mm, which closes to the diffraction limited. The system has compact constructure and has high imaging quality. The feasibility and rationality of the splicing principle were verified using prototype .