A nitrogen heterocyclic compound having a structure of Chemical Formula I is provided. z is 1 or 2, m and n are each 1 or 2, p and q are each 0, 1 or 2. X1-X6 are each independently nitrogen or carbon, and at least one of X1-X6 is nitrogen. R1, R2 and R3 are each independently C1-C10 linear or branched alkyl, substituted or unsubstituted aryl, fused aryl, an aromatic heterocyclic group, or a fused aromatic heterocyclic group. R1 and R2 may be otherwise a single bond. Ar1 and Ar2 are each substituted or unsubstituted aryl, fused aryl, an aromatic heterocyclic group, or a fused aromatic heterocyclic group. The nitrogen heterocyclic compound has a higher refractive index, can improve light extraction efficiency and luminous efficiency and alleviate angular dependence of luminescence when it is used as a capping layer CPL of an OLED device.