To accurately locate the focal plane of object in digital holography, the overall sharpness (OS) method, which is the supplement of the edge-sharpness (ES) method, is presented. By combining above methods, the integrated gray-level gradient method is obtained. The criterion with ES is confirmed to be appropriate for large particles or small object distance. In contrast, the criterion with OS is determined to be appropriate for small particles or large object distance. The conditions of above criteria are discussed by using numerical simulation holograms of particles. At last, the integrated gray-level gradient method is applied to the real holograms of dot array target and calibration target. The experimental results demonstrate that this method can effectively extract the focal plane of particle.