Provided is a polyisocyanurate foam formed by polymerizing and foaming a composition comprising: a polyol-based compound, an isocyanate-based compound and a solid flame retardant, wherein a viscosity of the polyol-based compound is 1,000 cps to 100,000 cps at 25° C. In addition, provided is a preparation method for a polyisocyanurate foam, comprising: (a) mixing and stirring a polyol-based compound having a viscosity of 1,000 cps to 100,000 cps at 25° C. and a solid flame retardant; and (b) mixing and stirring the mixture obtained from step (a) with an isocyanate-based compound to polymerize and foam polyisocyanurate.