Abstract: In order to improve the crack resistance and adhesion with organic matrix of geopolymer cementitious materials, a novel phosphorus-containing organic polysiloxane (PDV) was synthesized using 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and vinyl triethoxysilane (VTES) as raw materials for the modification of geopolymer cementitious materials. The prepared organic-modified geopolymer cementitious materials were then coated on the surface of the rigid polyurethane foam (RPUF) as a fireproof coating. The effects of PDV content on the surface drying time, crack resistance, and adhesive strength of geopolymer cementitious materials were studied. In addition, the influences of thickness of organic-modified geopolymer fireproof coating on the flame retardant performance and combustion behavior of RPUF were investigated. The results show that the introduction of PDV shortens the surface drying time of geopolymer coating, and is beneficial to improve its crack resistance and adhesive strength with RPUF matrix. The organic-modified geopolymer coating can endow RPUF with flame retardancy/fire resistance, and effectively inhibit the heat release rate and smoke production rate during combustion. Specifically, the coated RPUF (400 μm) can achieve UL-94 V-0 certification, and its limit oxygen index (LOI) is greater than 48%. With the increase of coating thickness, the char residue of the prepared samples gradually increases. Importantly, the introduction of PDV has no significant impact on the thermal degradation behavior of geopolymer cementitious materials.