Mechanisms of deep-focus earthquakes have puzzled generations of scientists since their discovery in the 1920s by Wadati. Deep earthquakes constitute more than 20% among earthquakes with magnitudes greater than 5. Because of their common occurrences, their association with subduction zones and their important role in studying the interior structure of the Earth, mechanisms of deep earthquake are of great interest. To explain the occurrences of deep earthquakes, a variety of mechanisms has been proposed. This paper would firstly introduce the properties of deep earthquakes which provides insight into mechanical nature, such as moment tensors and focal mechanisms. Secondly, the paper would review the most commonly accepted mechanisms of deep earthquake occurrence, including linear-vector dipole, polymorphic phase transformation, shear-induced melting, and plastic instabilities. At last, some unanswered questions that deserve future research attention would be summarized.