Lithium borohydride (LiBH4) is a promising hydrogen storage material due to its large effective hydrogen storage capacity of 13.6wt%. However, the high thermodynamic stability resulting in high hydrogen absorption and desorption temperature, poor hydrogen absorption and desorption kinetics, and harsh reversibility conditions seriously limit its practical application. In response to these problems, two kinds of Li-Al-B-H composites (LiBH4+0.5Al and LiBH4+0.5nano-Al) were prepared by mechanical ball milling method using ordinary Al powder (written as “Al”) and nanoscale Al powder (written as “nano-Al”) as modification additives. The microstructures and hydrogen desorption properties of the Li-Al-B-H composites prepared with different Al powders were studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), hydrogen absorption and desorption tests, and thermal analysis. The hydrogen desorption performance study shows that the hydrogen desorption temperature of LiBH4 is reduced after adding Al, and the L-Al-B-H composite constructed with nanoAl has better hydrogen releasing performance and cycling performance than the Li-Al-B-H composite constructed with ordinary Al powder. The microstructure study of the hydrogen desorption product shows that during the hydrogen desorption process, LiBH4 reacts with Al to form AlB2 and Li-Al-B phases, which is the key to the improvement of the hydrogen absorption and desorption performance of LiBH4. In addition, due to the smaller particle size, the nanoAl has larger specific surface area and more reaction interfaces, which leads to that the amount of AlB2 and Li-Al-B phases generated after LiBH4+0.5nano-Al dehydrogenation is more than that of LiBH4+0.5Al. This is the reason why the hydrogen releasing performance of LiBH4+0.5nano-Al is better than that of LiBH4+0.5Al. This work provides an important reference for further understanding the hydrogen storage performance and hydrogen storage mechanism of Li-Al-B-H hydrogen storage materials.