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The use of the Jonhson−Mehl−Avrami−Kolomogorov (JMAK) model has become prevalent in the kinetic analysis for Mg-based hydrogen storage alloys. However, the classical JMAK model has shown inadequate results in certain cases, resulting in poor fitness and/or inexplicably low Avrami exponent n. To address this issue, this paper proposes a new model based on the classic JMAK model. The new model considers the preferential growth of the hydride phase from the surface into the bulk, resulting in a diffusion barrier effect during the hydrogenation of Mg-based hydrogen storage alloys. A new parameter, surface diffusion barrier index b, is introduced to quantify the surface diffusion barrier effect. The governing equation is expressed as lnThe modified JMAK model demonstrates improved fitting performance for the hydrogenation analysis in multiple Mg-based systems. Additionally, the relationship between diffusion barrier index b and the reaction rate is discussed. The results suggest that the faster initial reaction rate leads to a stronger diffusion barrier, which inhibits the subsequent hydrogenation for the alloy.