Focusing of an off-axis telescope with the secondary mirror is studied based on a hexapod positioning system and a linear stage. That have compared linear positional focusing, rotating focusing and a combination of them (the hybrid method), and concluded that the linear positional focusing achieves higher MTF and lower OPD at short focusing distances, the rotating focusing performs well at medium and large focusing distances, while the hybrid method could be applied at all distances. A 800mm-diameter off-axis telescope is designed using the above methods, and the optical qualities at distances ranging from 0.6km to 10km are evaluated. Results show that the hybrid method leads to the highest imaging qualities.