The three-dimensional magnetohydrodynamic code JOREK was utilized to perform a linear study on the HL-2A device. The primary objective of this investigation is to gain insight into how the Resonant Magnetic Perturbation (RMP) influences plasma equilibrium and, consequently, the behavior of the Edge Localized Modes (ELMs). A scan of the (n is the toroidal mode number) RMP coil current from 1 to 4.9 kAt reveals a mode transition from to for the dominant toroidal mode number, after the RMP field exceeds a certain threshold. By comparing the changes in the electron density expansion diagrams at specified magnetic surface locations before and after the transition, it is found that the periodic structure resulting from the application of RMP forms an mode structure through mode linear coupling with natural ELMs (with the dominant mode number ). This is a key reason for the change in the dominant mode number.