The high-pressure phase diagrams of binary Se-N system have been constructed using the CALYPSO method and first-principles calculations. Four stable compounds ( Cmc21-SeN2, P21 /m-SeN3, P1̅-SeN4, and P1̅-SeN5) were identified at high pressures. Various peculiar nitrogen polymerization forms composed of single/double nitrogen-nitrogen bonds were found at the nitrogen-rich condition, such as N∞-chains in P21/ m-SeN3, oligomeric N8-chains in P1̅-SeN4, and distorted N63- anion rings in P1̅-SeN5. Peculiar nitrogen polymerization forms make these compounds potential high-energy-density materials (HEDMs). Especially, P1̅-SeN5 has the highest energy density of 4.08 kJ g-1 among the selenium nitrides. The polymerization mechanism of nitrogen in the Se-N system has been explored using the "Lewis-like" two-center-two-electron and three-center-two-electron bonding analysis. Using the nitrogen-rich P1̅-SeN5 as a prototype, it is found that the famous N6 distortion in the polymerized nitrogen HEDM can be explained by the interatomic mechanical unbalance which is induced by the three-center two-electron bonding between the metal atom and the two neighboring nitrogen atoms.