Abstract: This paper defines a complete layered architecture of cluster system software named Phoenix from a perspective of operating system's view, including three layers: heterogeneous resources, cluster operating system kernel and user environment. According to the core requirement of different user environment, the components of cluster operating system kernel and especially their correlations are described and defined. The scalable and fault-tolerant characteristic of cluster OS is guaranteed on the basis of the improvement of group structure. The Phoenix system has been installed on Dawning 4000A used for system monitoring, system administration and job management, and presently it supports Linux, AIX, Windows and Solaris operating system.