The invention discloses a power transmission line equivalent icing thickness calculation method, which comprises the steps of: step 1, extracting historical tensile force value measured data of an icing monitoring terminal, removing tensile force value data when a line is iced, and acquiring historical tensile force value data in a non-icing period; step 2, calculating a historical tensile force average value of the line when the line is not iced; step 3, calculating a tensile force value of the line when the line is not iced according to a conductor state equation, comparing the tensile force value with the historical tensile force average value when the line is not iced, and correcting tensile force value measured data; step 4, and establishing a power transmission line equivalent icing thickness calculation model which takes the line designed ice thickness and the vertical span varying along with the icing thickness into account according to the corrected tensile force value data acquired in the step 3. The power transmission line equivalent icing thickness calculation method solves the technical problems of large power transmission line equivalent icing thickness errors calculated by adopting the prior art and the like.