The problem of composition prediction of petroleum fractions was studied. Molecular connectivity index 1 X was used to describe the molecular structure of hydrocarbons Relationships between some state parameters of SRK (Soave-Redlich-Kwong) equation and 1 X were established. On the basis of these relationships, a theoretical model for predicting the composition of petroleum fractions according to their properties was proposed. Correctional Powell method was used to solve the model. Computational results of five examples indicate that the compositions of mam components agree well with the measured data, which prove the feasibility and reliability of the model. The problem of composition prediction of petroleum fractions was studied. Molecular connectivity index 1 X was used to describe the molecular structure of hydrocarbons Relationships between some state parameters of SRK (Soave-Redlich-Kwong) equation and 1 X were established. On the basis of these relationships, a theoretical model for predicting the composition of petroleum fractions according to their properties was proposed. Correctional Powell method was used to solve the model. which Computational results of five examples indicate that the compositions of mam components agree well with the measured data, which prove the feasibility and reliability of the model.