A high availability method for improving utilization of physical servers in a cloud computing resource pool, the method comprises: when the physical servers in the cloud computing resource pool fail, judging whether idle memory of the normally-running physical servers can support running of virtual machines of all the failed physical servers with reserved memory, wherein reserved memory is the minimum memory necessary for the running of a virtual machine; when the idle memory of the normally-running physical servers can support the running of the virtual machines of all the failed physical servers with the reserved memory, restarting the virtual machines of all the failed physical servers on the normally-running physical servers with the reserved memory; characterized in that, when the idle memory of the normally-running physical servers can not support the running of the virtual machines of all the failed physical servers, decreasing the memory occupied by the virtual machines on the normally-running physical servers, and sequentially using the decrease of the memory as the idle memory; wherein said decreasing the memory occupied by the virtual machines on the normally-running physical servers comprises: closing the virtual machines with the memory retention rate being zero configured on the normally-running physical servers; wherein the memory retention rate is the reserved memory divided by the configured memory, and wherein configured memory is the memory allocated for virtual machines when the virtual machines are created on physical servers, and changing the virtual machines with the memory retention rate being less than 1 and greater than 0 configured on the normally-running physical servers from running with occupying configured memory to running with occupying reserved memory. when the physical servers in the cloud computing resource pool fail, judging whether idle memory of the normally-running physical servers can support running of virtual machines of all the failed physical servers with reserved memory, wherein reserved memory is the minimum memory necessary for the running of a virtual machine; when the idle memory of the normally-running physical servers can support the running of the virtual machines of all the failed physical servers with the reserved memory, restarting the virtual machines of all the failed physical servers on the normally-running physical servers with the reserved memory; characterized in that, when the idle memory of the normally-running physical servers can not support the running of the virtual machines of all the failed physical servers, decreasing the memory occupied by the virtual machines on the normally-running physical servers, and sequentially using the decrease of the memory as the idle memory; wherein said decreasing the memory occupied by the virtual machines on the normally-running physical servers comprises: closing the virtual machines with the memory retention rate being zero configured on the normally-running physical servers; wherein the memory retention rate is the reserved memory divided by the configured memory, and wherein configured memory is the memory allocated for virtual machines when the virtual machines are created on physical servers, and changing the virtual machines with the memory retention rate being less than 1 and greater than 0 configured on the normally-running physical servers from running with occupying configured memory to running with occupying reserved memory. closing the virtual machines with the memory retention rate being zero configured on the normally-running physical servers; wherein the memory retention rate is the reserved memory divided by the configured memory, and wherein configured memory is the memory allocated for virtual machines when the virtual machines are created on physical servers, and changing the virtual machines with the memory retention rate being less than 1 and greater than 0 configured on the normally-running physical servers from running with occupying configured memory to running with occupying reserved memory.