1. A method for migrating virtual assets over a network having a first manager and a physical host connected, the physical host having a virtual machine run thereon, the method comprising: registering the physical host to a second manager in the network; creating a mapping relationship for the physical host between a database of the first manager and a database of the second manager, wherein the mapping relationship of the physical host in the two databases is based on records of the physical host in the databases of both the first manager and the second manager; importing instance data and status data of the virtual machine of the physical host from the database of the first manager into the database of the second manager; switching management for the physical host from the first manager to the second manager, wherein creating the mapping relationship of the physical host comprises: extracting an identity information of the physical host; locating a record of the physical host in the databases of both the first manage and the second manager based on the identity information; and establishing the mapping relationship of the physical host in the two databases according to the located record.