Molecule hydrogen (H₂) has been used to suppress tumor growth. To employ the H₂ therapy, it is necessary to use a proper agent for continuous generation of H₂. As a biodegradable metal, magnesium (Mg) generates H₂ in an aqueous environment, but the H₂ release rate is still too low. Here, we design a Mg-Al-Ca (AX) alloy that degrades very rapidly due to the presence of a secondary phase Al₂Ca. Having a reduction potential much higher than Mg and any other Mg-based secondary phases, Al₂Ca accelerates the corrosion of the Mg matrix by a micro-galvanic process. Al₂Ca also enhances the strength and ductility of the AX alloy. AX alloy rods show better anti-tumor efficacy than pure Mg rods in vivo. Moreover, implanted AX alloy rods can be heated under an alternating magnetic field to suppress large-size tumors. This work suggests that the H₂ therapy using highly degradable Mg alloys may provide an effective cancer treatment.