We obtain the isomer spectra of C30 and C31 clusters by time-going-backward quasi-dynamies method and perform molecular dynamics simulations of the duster growth from isolated atoms jn He buffer gas at 2500K.The geometrical structures of the isomers of C30 and C31 can be classified into closed cages,open cages,bowls,sheets and other irregular shapes,where closed cages are found to have the lowest potential energies.However,dynamics simulatious show that the sheet structures of C30 and C31 are the dominant outcome at the simulation temperature.Compared with relevant experimental results.we propose a different view in interpreting the experimental data and a research procedure to predict isomers that would be formed most probably under specific experimental conditions.