The geometry, the frontier molecular orbital and the charge transfer property of lithium (Li) doped tris (8-hydroxyquinoline) aluminum (Alq3) are investigated by the density functional theory (DFT) method. The calculated results show that in the Li-doped Alq3 system, Li-N and Li-O bonds are formed and Li-Alq3 electron transfer complexes are obtained. The incomplete electron transfer from Li atoms to the pyridine side of Alq3 results in donor level in the band gap of Alq3, a typical n-type doping formation, which can improve efficiently the electron transport efficiency. However heavy Li doping can induce the dissociation of Alq3, which leads to the decline of electron transport ability. When the Li:Alq3 doping ratio is about 2:1, the Li-doped Alq3 layer will have a maximal electron transport efficiency.