Simulation of Ni K-edge X-ray absorption near edge structure (XANES) spectra in LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) was performed. The structure of NCM523 was optimized by first-principles calculation based on density functional theory and XANES spectrum simulation via the finite difference method. The calculated Ni K-edge XANES spectra of NCM523 with Li amounts of 1.0 and 0.5 showed good agreement with the measured spectra. The bond length between Ni and O shortened as the valence of Ni increased. Distortion of the structure resulting from Jahn–Teller distortion was observed with Ni³⁺. The XANES spectra of the Ni K-edge of Ni²⁺, Ni³⁺, and Ni⁴⁺ were calculated. In NCM523 with a Li amount of 1.0, the spectrum of Ni³⁺ shifts towards the higher energy side compared to that of Ni²⁺; at a Li amount of 0.5 the absorption edge of Ni²⁺, Ni³⁺, and Ni⁴⁺ shifts toward a higher energy as valence increases. Even at the same Ni valence number, the XANES spectra were different when the Li amounts were 1.0 and 0.5. Cation mixing of Li/Ni readily occurs at a Li amount of 1.0, more than that of 0.5 because of the super exchange interaction. The K-edge XANES spectrum of the Ni of the Li site did not change the position of the absorption edge of the Ni site Ni²⁺ XANES spectrum; a difference in shape of the shoulder peak and the pre-edge peak appeared. From these results, the Ni valence, bonding state, and cation mixing effect of Li/Ni on the Ni K-edge XANES spectrum in NCM523 were clarified.