A thermally perfect overall reaction model is proposed to reduce computational load for detonation simulations. The proposed model is consisted of three gas components modeled as thermally perfect gas, which are completely premixed detonable mixture, burned gas and inert gas. For chemical reactions, it is considered that detonable mixture reacts to burned gas under the one-step irreversible chemical reactions. The thermodynamic data for each gas component correctly follows enthalpy changes of the mixture gas per unit mass, and it is assumed that burned gas is isentropically expanded. A single-cycle and multiple-cycle operations of one-dimensional and two-dimensional single-tube Pulse Detonation Engine were investigated with the proposed model. The pressure and temperature histories at the closed and open ends and the x-t diagram of temperature distribution are compared with the results obtained by the detailed and the one-step chemical reaction models. The simulation results by the proposed model agree well with those of the detailed model. The proposed model reduces the CPU time to about 9% of the detailed model for one-dimensional analysis, and 14% for two-dimensional analysis. Thus, computational cost can be substantially decreased by using the proposed model.