The invention discloses a method for preparing L-theanine by a chemical method. The method comprises the following steps: firstly, reacting ethylamine with acryloyl chloride in the presence of an alkali to obtain N-ethylacrylamide; secondly, perform Michael addition reaction on chiral auxiliary BPB based glycine Schiff base Ni(II) coordination compound and the N-ethylacrylamide in the presence of the alkali to obtain theanine Schiff base Ni(II) coordination compound; and finally, performing acid hydrolysis on the theanine Schiff base Ni(II) coordination compound to prepare the L-theanine. The method adopts the chiral auxiliary chemical method to prepare the L-theanine, and constructs a complete theanine fragment through the Michael addition reaction so as to prepare the theanine Schiff base Ni(II) coordination compound, wherein the rigid space structure of the coordination compound can maintain the L-configuration of the theanine so as to ensure the optical purity of the L-theanine. The method can reclaim the chiral auxiliary in high yield, and can elute Ni(II) ions through ion exchange for recycling.