Disclosed are an organic electroluminescent device and a preparation method thereof. The organic electroluminescent device comprises an anode, a hole transport layer, an organic light-emitting layer, an electron transport layer and a cathode. An organic metal complex and an active metal compound are doped in the electron transport layer, wherein the active metal compound is an alkali metal complex, an alkali earth metal complex or a lanthanide metal compound. The preparation method thereof includes the following steps: etching an anode pattern, and evaporating a hole transport layer and an organic light-emitting layer on an ITO glass substrate in order; and co-evaporate an electron transport material, an organic metal complex and an active metal compound to form an electron transport layer; and evaporating a cathode on the electron transport layer.