An adaptive mountain-climb searching strategy based on No-Reference Structural Sharpness (NRSS) of single image is proposed for autofocus system. This new method utilizes the prior knowledge on defocusing amount provided by NRSS. If the imaging system is greatly defocused, a large step-size is used to approach the focusing position quickly; otherwise, a small step-size is used to locate the focusing position accurately. This method has the same focusing accuracy to mountain-climb strategy, and it greatly decreases the number of images and the number of decision makings that are required. The experimental results indicate that the proposed method improves the speed of autofocus about 50%.