The invention discloses a long-focal-length large-view-field miniaturized active athermalization optical system which comprises a Cassegrain reflection primary mirror, a reflection secondary mirror, a field lens, a collimating mirror, a first folding mirror, a second folding mirror, a first imaging mirror group, a third folding mirror, a second imaging mirror group, a fourth folding mirror, an imaging lens and a detector focal plane. The Cassegrain reflection primary mirror and the reflection secondary mirror form a Cassegrain imaging system, the field lens is arranged between the Cassegrain reflection primary mirror and the reflection secondary mirror, and the field lens is used for compressing the field angle of the collimating mirror and correcting the field curvature of the imaging system; and the collimating lens, the first folding lens, the second folding lens, the first imaging lens group, the third folding lens, the second imaging lens group, the fourth folding lens, the imaging lens and the detector focal plane are sequentially connected through light paths. The system can work in a wide temperature range, the cost of the system is reduced, meanwhile, the space structure of the system is optimized through the turning mirror, and miniaturization is achieved.