The invention belongs to the field of chemical power sources and also belongs to the technical field of energy materials, in particular to a preparation method and application of a metal lithium battery with a sandwich structure and long life. According to the method, metal lithium tightly fits film capable of being formed through an in-situ reaction and being conductive to metal lithium negativeelectrode cycle, in an assembly process, a protective layer is generated in situ through the addition of an electrolyte, and the thickness of the protective layer can be controlled through the amountof reactants in the film. The metal lithium is taken as the negative electrode, and recombination can be subjected to the in-situ reaction to generate the film with an effective protective layer and other structures. The composite structure can stabilize a metal lithium/electrolyte interface, inhibit the growth of lithium dendrites to some extent, and improve the electrochemical performance of themetal lithium.