Tetradentate Pt(II) complexes, comprising of benzoimidazolyl N-heterocyclic carbene (BI-NHC) coordination, have shown superior performance in blue phosphorescent organic light-emitting diodes (PhOLEDs). Here, four frame-structured complexes were synthesized to explore the intrinsic ligand effect for rapid, efficient and narrow-spectra blue phosphorescence through experimental analyses and theoretical calculations. We found carbazolylpridine (CzPy) chelating part in Pt(II) BI-NHC complexes dominates the phosphorescence with the suppression of metal-centered transition and electron-vibration coupling. Consequently, the CzPy chelated complexes (PtN-dip and PtN-dtb) achieved high luminescent efficiency, rapid radiative decay rate (kr) and narrow emission spectra. The peripheral substituent 2,6-di-iso-propylphenyl (dip) on NHC of PtN-dip constrains the molecule, significantly reducing the non-radiative decay rate (knr) and enhancing the efficiency. The 3,5-di-tert-butylphenyl (dtb) in PtN-dtb could enhances low-frequency vibration coupling that could expedite the excitons decay, resulting in higher kr and knr. This work unveils the intrinsic factors behind the ligand effects of Pt(II) BI-NHC based complexes, leading to a rapid, efficient and narrow-spectra phosphorescence.