In order to investigate the influences of laser pulse energy on the radiation characteristics and the expansion process of laser-induced plasma, the fast images of laser-induced aluminum alloy plasma were obtained by ICCD camera under different laser pulse energy, and the electron temperature and electron number density with different laser pulse energy were also determined by using the Boltzmann plot method and Stark broadening of line, respectively. The results show that the plasma presents an obvious hierarchical structure, and the excitation threshold of the plasma is approximate 3 mJ. The areas of different regions in plasma present different characteristics with different laser pulse energy. There is no significant hierarchical structure when the laser pulse energy is under 10 mJ. The electron temperature increases from 4 980 K to 7 221 K, and the electron number density which is of the order of 1017 cm－3 increases firstly and then trends to saturation when the laser pulse energy increases from 10 mJ to 100 mJ.