A novel atomic force microscope (AFM) for large samples to be measured in liquid is developed. An innovative laser beam tracking system is proposed to eliminate the tracking and feedback errors. The open probe design of the AFM makes the operation in liquid convenient and easy. A standard 1200-lines/mm grating and a sheet of filter paper are imaged respectively in air and liquid to confirm its performance. The corrosion behavior of aluminum surface in 1-mol/L NaOH solution is further investigated by the AFM. Experimental results show that the system can realize wide range (20 × 20 (\mu m)) scanning for large samples both in air and liquid, while keeping nanometer order resolution in liquid by eliminating the tracking and feedback error.