A freeform off-axis three-mirror system based on a freeform mirror was designed for the long focal length and wide-field off-axis three-mirror system in a space telescope. To obtain a system with a focal length of 4 500 mm, field of view of 11° and the ratio of the total length of system and the focal length in 1/3, the design and performance between traditional off-axis three-mirror system and freeform off-axis three-mirror system were compared. In the designed optical system, the freeform mirror was introduced to the secondary mirror to enhance system optimization and improve the balance capacity for optical aberration. After the comparison and optimization, the relative aperture of 1/9.5 was chosen.In the system, the average of wavefront error of all field of view is better than 0.030λ(λ=632.8 nm), and the average of the Modulation Transform Function(MTF) of all field of view is greater than 0.434(71.4 lp/mm) that is close to the diffraction limitation. Furthermore, the MTF has im-proved more than 5% compared to that of traditional TMA system and the departure of the secondary freeform mirror and theoretical spherical surface is only 1.1λ(λ=632.8 nm). The highly precision testing can be accomplished only by a standard sphere combined with the non-null testing method based on digital mask, which solves the problem of large-aperture convex freeform mirror testing and is helpful for the manufacturing of freeform secondary mirror. The off-axis three-mirror system based on freeform secondary mirror shows its advantages in smaller cubage, better realizability in engineering, better wavefront error and higher MTF.