The present invention discloses an ultra-small-sized 4K-resolution ultra-short-focus projection optical system, which is characterized by including in sequence in a projection direction: a DMD chip, an equivalent prism, a 4K oscillating mirror, a refraction lens assembly and an aspherical reflector. Through reasonable distribution of focal power, the semi-aperture size of the reflector is reduced to be less than 50 mm, and the assembling sensitivity is substantially reduced, so that batch production can be realized.