This paper reported the performance and mechanism of hydrogen generation via hydrolysis of ball-milled Mg-Mg2Si composite (5.3 wt % Si-94.7 wt % Mg) in deionized water and in MgCl2 solution. The results showed that the obtained Mg-Mg2Si composite presented relatively higher hydrogen generation performance than pure magnesium. Adoption of 0.5 M MgCl2 solution to replace deionized water sufficiently and vastly enhanced the hydrolysis properties of the Mg-Mg2Si composite. The composite in 0.5 M MgCl2 solution generated 445 mL/g hydrogen in 5 min, 688 mL/g hydrogen in 10 min and 889 mL/g hydrogen (conversion rate 99%) in 40 min at 328 K. This remarkable improvement is due to that the addition of Si element in the composite and the introduction of MgCl2 in solution, as well as the special preparation process of the materials, could decrease the formation of continuous magnesium hydroxide passive layer on the particle surface, directly or indirectly. Moreover, the apparent activation energies for composite hydrolysis in deionized water, in 0.5 and 2.0 M MgCl2 solution were calculated to be 30.1 +/- 0.6, 9.5 +/- 0.1 and 3.7 +/- 0.2 kJ/mol, respectively. This work demonstrates that the hydrogen generation system based on low-cost and high-performance Mg-Mg2Si composite is very applicable and promising; and it may open a new avenue for onsite hydrogen supply.