The invention discloses a method for calculating a service life of a battery energy storage system in a microgrid. The method is characterized by comprising the steps of analyzing corresponding relationships between charge-discharge depth of a lithium battery and a cycle index and between the charge-discharge depth of the lithium battery and a total throughput, and building a service life loss model of the battery energy storage system; and by adopting a fatigue life calculation method, namely, ''a tower top counting method'' in existing engineering field, performing charge-discharge cycle interception on the battery energy storage system, calculating the charge-discharge depth of the battery, and substituting the obtained charge-discharge cycles into the service life loss model for calculating the service life of the battery energy storage system. The method has the advantages of scientificity, reasonability, simplicity, applicability, high prediction precision and the like.