Nitrogen heterocyclic carbine ligands and ruthenium catalysts thereof, a preparation method therefor and an application thereof are provided. The structures of the nitrogen heterocyclic carbine ligands are represented by formulas Ia and Ib, respectively, and the corresponding ruthenium catalyst structures are represented by IIa and IIb, respectively. After simultaneously introducing large-steric hindrance and electron-rich groups into the described nitrogen heterocyclic carbine ligand structures, the catalytic activity, stability and application range of the ruthenium complex catalysts thereof are significantly improved.