Various factors affecting boiling state are analyzed when water flow and exponent-decadence of laser by water characteristic are considered. By using unit difference methods surplus temperature of the coverage by laser is studied. According to the relationship of surplus temperature and boiling state, forced Convective Boiling modal of water by laser is founded. Annular beam is regarded as the main research object, concerned data was simulated and experiments are carride out. As a result they have good coherence.