A resource adjustment system and method for virtual machines (VMs) are provided. The method includes: obtaining, by a monitoring module, performance and status information of a first server and at least one VM of the first server; estimating, by an estimating module, the first server and each VM according to the performance and status information and an evaluation index and obtaining a server estimation score of the first server and a virtual machine estimation score of every VM; and finding, by a moving module, a second server according to the performance and status information and the evaluation index when a virtual machine estimation score of at least one first VM of the first server or the server estimation score exceeds a threshold, finding at least one VM suitable to be moved according to a moving order, and moving the at least one VM to the second server.