<p>In conventional magnesium (Mg) alloy brazing, the high brittleness of intermetallic compounds (IMCs) in fillers often results in poor processability, significantly affects the reliability and accuracy of brazed joints. This work proposes a novel preparation method for Mg alloy filler, improving the processing performance of the filler and significantly increasing the strength of brazed joints. The Mg/Al filler preform was prepared by rolling composite, and the preform in situ formed Mg/Al filler by diffusion reaction, simultaneously achieving high-strength precision brazing of Mg alloys with a maximum shear strength of 76 MPa. This outstanding shear strength is attributed to the soft and hard dual-phase heterogeneous structure of α-Mg/Mg₁₇Al₁₂ alternating distribution formed during the brazing solidification. The joint performance is significantly improved through coordinating the stress and strain distribution. Simultaneously, rapid diffusion of Al during brazing led to the sequential formation of the Al₃Mg₂ and the Mg₁₇Al₁₂ within the preform. Subsequently, Mg₁₇Al₁₂ occurs eutectic melting with residual Mg, ultimately triggering the complete melting of the preform to achieve efficient and precise bonding. This work provides a novel method for preparing fillers and offers theoretical insights into the efficient brazing of Mg alloys.</p>