Ultra-short-focus projection optical system

The invention discloses an ultra-short-focus projection optical system, which is sequentially provided with a DMD chip, a refractive lens assembly and an aspherical reflecting mirror in the projection direction, and is characterized in that the refractive lens assembly comprises a first lens group capable of moving back and forth relative to the DMD chip, a second lens group capable of moving back and forth relative to the DMD chip, a third lens group capable of moving back and forth relative to the DMD chip and a fourth lens group being static relative to the DMD chip which are sequentially arranged along the projection direction, the focal power of the first lens group is positive, the focal power of the second lens group is negative, the focal power of the third lens group is positive, and the focal power of the fourth lens group is positive. The ultra-short-focus projection optical system is small in volume, high in resolution, less than 0.2 in projection ratio, capable of not defocusing at a high tempera