The invention provides a preparation method of a lithium metal negative pole and a lithium iron phosphate battery. The preparation method comprises the following steps: (1) dispersing aluminum salt and lithium salt into an organic solvent to obtain a mixed solution of the aluminum salt, the lithium salt and the organic solvent; (2) soaking a pretreated lithium metal sheet into the solution obtained in the step (1), soaking for a period of time, taking out and drying to obtain the lithium metal negative pole. According to the preparation method disclosed by the invention, by soaking the lithiummetal sheet in the solution prepared from the aluminum salt and the lithium salt, a layer of protective film is formed on the surface of the lithium metal sheet, thus even deposition of lithium can be obviously stabilized and formation of lithium dendrites is favorably inhibited. In addition, a preparation technology is simple, easy to adjust and control; conventional equipment is used, raw materials are easy to obtain, and the cost is low; the preparation method is safe, pollution-free and is suitable for mass production.