The present invention relates to an n-heterocyclic carbene (NHC) type palladium catalyst and its preparation method as well as applications. Its preparation process is as below: select glyoxal as the raw material to synthesize glyoxaldiimine in the presence of Lewis acid or Bronsted acid, and then react with paraformaldehyde to get the NHC type ligand. Use palladium(II) to react with the compound containing carbon-nitrogen double bonds to get palladium(II) cyclic dimer; make the palladium cyclic dimer and the NHC type ligand coordinated to get the NHC type palladium catalyst. The palladium catalyst with a brand new structure according to the present invention, boasts high activity and multi-purpose. In addition, it shows excellent reaction activity in a lot of catalytic-coupling reactions including Suzuki-Miyaura, Heck, Buchwald-Hartwig, Kumada-Tamao-Corriu, Sonogashira, Negishi and α-ketone arylation reactions, and some reactions even can be carried out with the presence of an extremely low concentration of catalyst, exhibiting favorable industrialization prospect.