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We propose a new CaN4 high pressure structure with the P21/m space group.The P21/m-CaN4 structure is constituted by the infinite armchair N-chain.The dynamical stability and mechanical stability are verified by the calculations of phonon dispersion curves and elastic constants.The enthalpy difference calculation shows that the P21/m phase is more stable than the reported P41212 phase.The advantaged properties of P21/m-CaN4,such as high nitrogen content (58.3％) and low polymerization pressure (18.3 GPa),allow it to be a potential high energy material.Band structure calculation shows that the P21/m-CaN4 structure is a metallic phase.The nonpolar covalent single N-N bond is a sigma bond.The charge transfer between the Ca and N atoms results in an ionic bond interaction.