The invention discloses an organic electroluminescent device and a preparation method thereof. The organic electroluminescent device comprises an conductive anode, a hole transport layer, a light emitting layer, an electron transport layer and a cathode layer; wherein the conductive anode, the hole transport layer, the light emitting layer, the electron transport layer and the cathode layer are successively laminated. The electron transport layer is made of a mixed material which is composed of organic lithium, a metal element and an organic electron transport material. The organic lithium is 4-hydroxyl phenanthridine lithium, 2-(5-phenyl-1,3,4-oxadiazole)-phenol lithium, 2,3-diphenyl-5-hydroxyquionline lithium, lithium tetra(8-hydroxyquinolinato)boron, tert-butylcyclopentadienyl lithium or lithium acetylacetonate. The metal element and the cathode layer are made of same materials, namely silver or aluminum. The organic electrode transport material is 8-hydroxyquinoline aluminum, 4,7-diphenyl-phenanthroline, 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene or 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline. The organic electroluminescent device has long service life and relatively high light emitting efficiency.