At present, the commonly used auto focusing methods for digital microscopes are divided into active auto focusing and passive auto focusing. Active auto focusing relies on a specific sensor or a distance measuring device, which automatically emits light and receives reflected light to determine the distance from the object to the focusing device and achieve focusing. Passive auto focusing can determine the focus direction and focus position by analyzing the image information of each position, therefore realizing focusing. Passive auto focusing is the most commonly used auto focusing method for digital microscopes due to its low structure requirements and low cost. This paper mainly introduces the auto focusing algorithm for digital microscopes, briefly describes the principle and development direction of active and passive auto focusing methods, focuses on the depth from focus method of passive auto focusing methods, and analyzes two key technologies of the depth from focus method, that is, the sharpness evaluation function and its focus search algorithm.