By calculating the normal heat of formation (HOF) of dinitro-polyfurazan series in gas state,an effective method was obtained to calculate the HOF of larger molecules (the relative error is below 7%). The method includes the following processes: (1) designing reactions which have equal bonds and equal electronic pairs，(2) obtaining the total energy (including ZPVE) and the heat of reaction by using parallel calculations (such as parallel Guassian,Dmol³)，(3) getting the HOF of small molecules without experimental value by accurately calculating energy (such as G2, CBS-4), and the HOF of unknown molecules by using the experimental values of HOF of the related molecules and calculating the energy. Moreover, the method can be used in other systems.