A method and a computer system for memory management on a virtual machine system are provided. The memory management method includes the following steps. First, a working set size of each of a plurality of virtual machines on the virtual machine system is obtained by at least one processor, wherein the working set size is an amount of memory required to run applications on each of the virtual machines. Then, an amount of storage memory is allocated to each of the virtual machines by the at least one processor according to the working set size of each of the virtual machines and at least one swapin or refault event, wherein the storage memory is a part of memory available from the computer system.