Adaptive optics scanning laser ophthalmoscopy (AOSLO) has gained increased research attention over the past few decades. This study proposes a focus-adjustable AOSLO that uses a novel optical design comprising a Badal system, a deformable mirror, and a Shack-Hartmann wavefront sensor. This design corrects the low-order aberration of the human eye by setting a target and adjusting the distance between lenses in the Badal system. The optimum parameters of the Badal system based on the focusing and working principle of the system are analyzed; then, the system is simulated and optimized by Zemax software. Simulation results show that the spot size of the proposed system is smaller than the diffraction limit in the spot diagram. The Strehl ratio of any diopter is more than 0.8 and approaches as high as 0.95 for a low diopter. The modulation transfer function of every field approaches the diffraction limit. Moreover, the theoretical resolution in the normal human retina of this system (2.29 μm) is in a close range to the diffraction-limit resolution of the proposed system (2.11 μm). The proposed system can achieve a clear imaging of the human retina for people with a diopter from -6 m -1 to +6 m -1.