This paper mechanisms of halo coronal mass ejections (halo CMEs) have been mainly presented, including how the evolution of vector magnetic field triggers the halo CMEs? What is the relation between the halo CME, and the activities of flares and filaments? Is there any pattern available? How the filament explosion and flare activity are related to each other? Is the halo CME event driven by either an activity region or a single activity event, or resulted from interaction among either different activity regions or among different activity events. Two observational evidences to the solar surface origin of halo CME are demonstrated.  It is suggested the cancellation of two opposite magnetic flux is the main characteristics of the evolution of magnetic fields in the solar surface areas.