Magnesium hydride (MgH 2 ) is one of the most promising candidates for hydrogen generation materials due to the benign byproduct Mg(OH) 2 . However, the low solubility of Mg(OH) 2 inevitably causes sluggish hydrolysis kinetics and low hydrogen yield. Herein, a facile strategy is reported to promote the hydrolysis performance of MgH 2 viain situ formation of metal borohydrides (M(BH 4 ) x ). MgH 2 @M(BH 4 ) x composites are synthesized by ball milling of MgH 2 and cheap metaborates. Different ball-milled durations of NaBO 2 metaborate and electronegativities (χ p ) of cation elements of metaborates (i.e., KBO 2 , NaBO 2 , LiBO 2 , Ca(BO 2 ) 2 , and Mg(BO 2 ) 2 ) were compared to reveal that the MgH 2 −4 mol % NaBO 2 composite after 6 h ball milling has the best hydrolysis performance. Specifically, it could release 1597.2 mL g −1 H 2 (95% theoretical H 2 ) in 60 min with a fast hydrolysis rate of 1216 mL g −1 min −1 H 2 during the first minute of the hydrolysis at 55 °C. Our finding is expected to facilitate the application of MgH 2 for on-site hydrogen generation.