The present invention provides a photovoltaic power station string fault diagnosis method based on a generation capacity cluster. The method comprises: the string generation capacity in the same combiner box is evaluated according to the excellent, medium and bad groups and is subjected to automatic cluster with no need for extra assigned parameters. The group which each string belongs to is recorded, the changing of the number of each group of strings is analyzed as time goes on, and the fault strings are located according to the characteristics. The photovoltaic power station string fault diagnosis method is simple and easy to deploy, can accurately locate the feature of the fault strings, and is high in practicality.