A kinetic model was proposed to describe fluid catalytic cracking of vacuum gas oil on a molecular level by the structure-oriented lumping method. The structure-oriented lumping method was applied to construct a feedstock matrix and reaction networks. All totaled, 391 kinds of molecules were selected to represent feedstock composition, and a non-linear least squares method was used to calculate the content of each kind of molecule. Sixty-six kinds of reaction rules were established to produce the networks for the reactions of vacuum gas oil. Product distribution and properties can be predicted by the structure-oriented lumping model, which are in good agreement with the experimental data.