A method for preparing an ultrathin Li complex includes the steps of preparing an organic transition layer on a substrate in advance, and contacting the substrate having transition layer with molten Li in argon atmosphere with H2O≤0.1 ppm and O2≤0.1 ppm. The molten Li spreads rapidly on the surface of the substrate to form a lithium thin layer. The ultrathin Li layer stores lithium on the current collector beforehand. It can be used as a safe lithium anode to inhibit dendrites.