In this study, the ablation of aluminum sheets under single pulse femtosecond laser with different energy densities was studied. To simulate data map by using finite difference method and study the relationship between laser energy density and ablation, the femtosecond laser ablation experiment results were combined with two-temperature model. The ablation process of femtosecond laser belongs to non-equilibrium ablation. According to the simulated data, the surface electron temperature and lattice temperature could be directly observed during the ablation process of aluminum sheets. The thermal relaxation in the whole laser-ablation process was furthermore studied. The electro acoustic interaction under femtosecond laser ablation with different energy densities was studied. The correlation between energy density and femtosecond laser ablation was revealed based on the simulated results. Through analysis of the experimental data, it could be concluded that the femtosecond laser energy density, the processing efficiency and the quality of the processed aluminum materials were the three significant factors. Moreover, it could be found that with the increase of femtosecond laser energy density, the thermal relaxation process of femtosecond laser ablation was elongated, the ablation strength was enhanced to some extent, and the appearance quality of the processed materials could be improved. Therefore, it is of great importance to study the femtosecond laser ablation of materials.