The multiphoton ionization (MPI) efficiencies and laser power indexes versus laser powers of different laser beam′s spatial and temporal structures were calculated for the three-level systems. The results were compared with those of the square wave approach. Based on the comparison, it was found that the laser beam′s temporal structure and spatial structure in the lower laser power region have very little effect on the laser power index, but the laser beam′s spatial structure exerts great influence on the multiphoton ionization processes in the higher laser power range. This analysis was supported by the experimental results.