1. A high availability (HA) method for improving utilization of physical servers in a cloud computing resource pool, wherein 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 on all the failed physical servers; when the idle memory of the normally-running physical servers can support the running of the virtual machines on all the failed physical servers, restarting the virtual machines on all the failed physical servers on the normally-running physical servers; wherein the method further comprises: when the physical servers in the cloud computing resource pool fail, judging whether the idle memory of the normally-running physical servers can support the running of the virtual machines on all the failed physical servers with reserved memory, and when the idle memory of the normally-running physical servers can support the running of the virtual machines on all the failed physical servers with the reserved memory, restarting the virtual machines on all the failed physical servers on the normally running physical servers with the reserved memory; when the idle memory of the normally-running physical servers can not support the running of the virtual machines on 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.