In this research, we designed an MWIR optical system, which is equipped with long focal length, large zoom ratio, axis-zoom and four fields of view. The optical system is composed of seven components, including a front fixing group, a zoom focus group, an intermediate compensation group, a rear fixing group, a first mirror, a second mirror and a relay group. The optical system uses optically compensation superposition mechanical compensation method to improve the major flaws existing in previous similar optical system, that is, the single optically compensation or mechanically compensated zoom optical system cannot meet the followed requirements at the same time. The requirements includes long focal length, large zoom ratio, compact optical system, and temperature compensation for optical system under wide temperature range (-40 to 70℃), etc. The improvement achieved long focal length, large zoom ratio, axis-zoom and four fields of view. The improved design results show that the image quality of the optical system is perfect, and it can meet the requirements of the thermal imager.