The present invention provides a kind of inherent flame retardant rigid polyurethane foam. The production formula comprises 100 to 105 pbw of polyether polyol and reactive phosphorus-containing flame retardant, 2.5 to 3.5 pbw of amine catalyst, 0.8 to 2.5 pbw of tertiary amine catalyst, 0.8 to 2.5 pbw of foam stabilizer, 0.5 to 1.5 pbw of blowing agent, 135 to 150 pbw of isocyanates, and 0.05 to 0.1 pbw of organo-metallic catalyst, wherein the reactive phosphorus-containing flame retardant is 9,10-dihydro-9-oxa-10-phosphaphenanthrene-4-hydroxybenzyl alcohol. The active monomers containing flame retarding elements are introduced into main chain and side chain of PU for modification, which permanently improves the flame retardancy of PU without obvious effect on other performance of PU matrix.