The invention discloses a power transmission line rainfall secondary geological disaster alarm calculation method and system, which belong to the technical field of electric engineering. The method comprises the following steps: obtaining historical rainfall data along a power transmission line, future predicted rainfall data and rainfall secondary geological disaster occurrence probability; according to the historical rainfall data along the power transmission line and the future predicted rainfall data, calculating future effective rainfall amount; according to the future effective rainfall amount and the rainfall secondary geological disaster occurrence probability index data, obtaining rainfall secondary geological disaster alarm indexes; and according to the rainfall secondary geological disaster alarm indexes, selecting an alarm level and making a treatment strategy according to the alarm level. For the current situations of lack of study of power transmission line rainfall secondary geological disaster impact situations and lack of analysis and summary of disaster alarm, the invention provides the power transmission line rainfall secondary geological disaster alarm calculation method and system; and the method and system are novel in ideas, clear in process and simple to operate.