With the increasing development of space technology, space optical system with the characteristics of high-performance, miniaturization has become a new hotspot research in space optical fields. Off-axis three-mirror optical system has the advantage of high quality image, large field of view and high level of lightweight etc., which can better suit the application of miniaturized and low-cost space optical system and has broad application prospects. Based on the Gaussian optics and three-mirror aberration theory, the off-axis three-mirror with freeform surface of tertiary mirror was designed. The focal length was 1 550 mm, field of view was 3.6°×0.45°, relative aperture was 1: 6.2, degrees of freedom and image quality were increased efficiently by the introduction of freeform surface. The design results show that system has a better performance in effective field of view, modulation transfer function value is above 0.43@111 lp/mm, wave-front error maximum value is 0.049 λ(λ=632.8 nm), RMS wave-front error value is 0.034 λ, maximum grid distortion value is 0.9%, and the imaging quality is complete symmetrical about the tangential plane. The total length of the optical system is less than f′/3.1, the height is less than f′/4.1, and is easily implemented because of the relatively loose tolerance about processing and assembling. The obtained results have a certain reference value for miniaturized space optical system.