A new kind of spacebased telescope with variable resolution at any field angle by active optical zoom is proposed theoretically. It can change the magnification of imaging system at any field angle that means the effective focal length can be changed at a limited region of interest and simultaneously maintain high resolution. It comprises two static aspheric mirrors and two deformable mirrors and the focal length can be adjusted from 399 to 558.6 mm by changing the curvature of the two deformable mirrors. Comparing with the mechanical zoom systems, they are active optical zoom systems, in addition, they avoid the precisely moving of elements and reduce the response time. The imaging quality of this optical zoom system is examined by analyzing the Spot Diagrams and the Modulation Transfer Function (MTF) curves at different field of view especially about the outside of the specific FOV. The Root Mean Square (RMS) radius of field angle at 0, 0.51 and 0.7 degrees are about 1.6 μm, 1.0 μm, and 1.7 μm, respectively, and the MTF values in the Spatial Frequency (SF) of 68 lp/mm is almost unchanged at 0.7. The novel system reduces the bandwidth of data transmission and may find potential applications in remote sensing.