Based on the principle of energy conservation in a cylinder test and the empirical formula of a cylinder's expansion velocity at feature distances, a simple method for calculating parameters of JWL EOS of detonation products of CHNO type explosives was presented. If the density and detonation velocity of the explosive were given, the parameters of JWL EOS of detonation products could be determined with this method. It did not require a cylinder test and it was more economical, convenient and accurate. Using this method, four kinds of common explosive were studied. By comparing to the P-V curve of JWL EOS obtained with a cylinder test and numerical simulations, it was shown that this method has a higher precision and meets the needs of explosion mechanics applications.