The invention relates to a method for calculating a frequent occurrence period of power transmission line mountain fire alarms. The method comprises: building fire point alarm vectors, adding all the fire point alarm vectors together to obtain a vector sum Fn of a fire point within a certain period, and calculating a frequent occurrence period index Ft (defined as in the description) of power transmission line mountain fire alarms in a region A, wherein [delta]t is a unit time period, n[delta]t is a total calculation time period, and [theta]n is an included angle between Fn and a horizontal coordinate axis; the norm of each fire point alarm vector is a total number F(A, k[delta]t) of power transmission line mountain fire alarms of the same period in history, and the direction of each vector is [theta]k (defined as in the description), wherein k=1, 2, ......, n. The method provided by the invention has extensive applicability and good transplantability. By using the method, frequent occurrence periods of mountain fire alarms of any region, different voltage levels and different power transmission line corridors can be calculated.