A series of vanadium catalysts with bulky bis(N-heterocyclic carbenes) (bis-NHCs) ligands were synthesized by in-situ reaction. With diethylaluminum chloride (AlEt2Cl) as cocatalyst and ethyl trichloroacetate (ETA) as activator, these catalysts exhibit good activity when used to catalyze ethylene (co)polymerization. The structure of the catalyst, especially the bridging carbons number of the bis-NHCs ligands, is related to the performance of the (co)polymer. When the bis-NHCs ligand with tBu substituted, increased the bridging carbons number of the bis-NHCs ligand is easier to obtain higher molecular weight polymers. But when the bis-NHCs ligand with 2,6-diisopropyl phenyl substituted, the increase of the bridging carbons number is not conducive to obtaining higher molecular weight polymers. Under the same concentration of comonomer, the increase of the bis-NHCs ligand bridged carbon number of the V-complexes is more beneficial to obtain the copolymer with high norbornene (NB) content.