A method, an apparatus, and a system for online migrating from a physical machine to a virtual machine are provided. The method including: after a target virtual machine is created, started, and suspended by a virtualization platform VMM Host, initially synchronizing data of a memory page from a source physical machine to the target virtual machine at a second time point; monitoring the operation of updating the memory page since the second time point; incrementally synchronizing data of the updated memory page in the source physical machine to the target virtual machine, and stopping monitoring when an increment value of the updated memory page in the source physical machine is less than a first threshold; and calling the virtualization platform VMM Host to resume the target virtual machine to a running state. The effect of smoothly switching services from the source physical machine to the target virtual machine is achieved.