By using a homemade laser induced breakdown spectroscopy system integrated with a micro-imager, the influence of aperture, region of interest, reflectivity of sample on position sensitivity of image acutance was analyzed. The results show that image acutance is more sensitive to the position under larger aperture, larger aperture is more suitable for the autofocus process. The reflectivity of sample has a little influence on the position sensitivity of image acutance, and the acutance of the partial image can also be used to assist the laser induced breakdown spectroscopy focus. The surface position corresponding to the maximum acutance has good repeatability for different samples and different region of interest. On this basis, an autofocus method based on the acutance of the surface image was established.