Porous graphene and Al modified graphene have been reported to be promising hydrogen storage materials owing to their possible high hydrogen storage capacity. In this work, Al-decorated porous graphene is considered as hydrogen storage material, based on density functional calculations. It is found that the hydrogen storage capacity of the Al-decorated porous graphene is 10.5 wt%, with modest hydrogen adsorption energy from -1.11 to -0.41 eV/H-2 to achieve efficient hydrogen storage/release at ambient conditions. In addition, hydrogen can be released gradually in three stages owing to different adsorption energy at different adsorption sites, which is desirable in actual hydrogen storage application. The mechanism for improving the hydrogen storage behavior by the decorated Al atom and its porosity is understood through analyzing the atomic charges, electronic distribution, and density of states of the system.