The invention discloses a preparation method of a polyurethane-polyisocyanurate thin-film graded material. The preparation method comprises the steps that aromatic diisocyanate and oligomer diol are used as raw materials, organic tin and Sudan blue are used as catalysts, a polyurethane-polyisocyanurate cross-linked network is prepared through urethane forming reaction and polycyclic trimerization reaction, and then a solution containing a reactor and the catalysts is poured into a hydrophobic mold to obtain a thin film through curing. Reaction mixture solutions different in proportion are poured onto a horizontal polycyclic surface, so that one surface of the prepared graded thin film is higher in elasticity modulus, the other surface of the prepared graded thin film is lower in elasticity modulus, the elasticity modulus in the thickness direction of the thin film changes from one surface to the other surface in a graded mode, no interface exists, the gradient change from soft rubber to rigid plastic of the prepared thin film in the thickness direction is presented macroscopically, the shortcomings of the brittleness and fragility of plastic thin films can be overcome, and the problem that the elasticity modulus of the rubber thin film changing with strain change can be also solved.