Abstract: The hybrid sky-surface wave system is a new HF radar system. The propagation mode associate with skysurface wave is nonlinear and timevarying channel, and its availability is analyzed. Then, based on radar equation, the environment influence on time availability of the system are analyzed. The study method and its calculated examples of the time availability of the hybrid skysurface wave radar which possesses given equipment capability and the target which possesses given radar cross section(RCS) are described. The influence of the time availability by the energy and RCS are pointed out. The study result is significant for the design project of the system.