The invention discloses an alkali activated phosphorus slag micropowder light foam concrete thermal insulation board, which comprises the following components by mass: 35-39% of phosphorus slag micropowder, 6-10% of an alkali activator, 7-12% of a coagulation regulating agent, 9-15% of a modifier, 0.4-0.8% of fiber, 9-12% of a foaming agent, and 22-26% of water. According to the invention, phosphorus slag micropowder is adopted as the main cementing material, through strong alkali activation and incorporation of appropriate coagulation regulating component and modification components, the coagulation time and shrinkage performance of foam concrete are controlled. The foam concrete thermal insulation board prepared by the method provided by the invention has an absolute dry density controlled at 150-250kg/m<3>, 28d compressive strength of 0.30-0.63Mpa, a heat conductivity coefficient of 0.042-0.051W/m.K, and a water absorption rate of 5-8%, thus being an ideal exterior wall external insulation wall enclosure material.