For the first time, a new flame-retardant formula based on dimethyl methylphosphonate (DMMP), aluminum hydroxide (ATH), ammonium polyphosphate (APP), and expandable graphite (EG) was applied to polyisocyanuratepolyurethane foams and found to exhibit a high flame-retardant efficiency and low cost, to be environmentally friendly, and to allow for the reduction of the amount of solid flame retardants added. The multiple effects were evaluated based on thermal conductivity tests, compressive strength tests, limiting oxygen index (LOI) measurements, cone calorimetry tests, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The results showed that ATH can form villi-like substances during the combustion process. In the presence of ATH and APP, dense spherical substances were produced. When EG was added to the system, a wormlike carbon layer that adsorbed phosphate-containing acid resulting from APP decomposition was formed during the decomposition process, so that the carbon layer was denser. The combined effects of the villi-like and spherical substances as well as the wormlike carbon layer can block heat and flame propagation from being transferred to the unburned foam.