Based on the principle of energy conservation in the cylinder test and the empirical formula of cylinder expansion velocity at feature distances, a simple method for calculating the parameters of JWL EOS of detonation products of CHNO explosives was presented. If density and detonation velocity of the explosive were given, the parameters of JWL EOS of detonation products could be obtained by this method. It does not require cylinder test and is more economical, convenient and accurate. Using this method, four kinds of common explosive have been studied. By comparing to the P-V curve of JWL EOS given by cylinder test and numerical simulation, we showed that this method has a high precision and meets the need of explosion mechanics application.