The invention discloses a power transmission line joint fault probability calculation method based on environment dependent failure. The power transmission line joint fault probability calculation method is characterized by considering uncertainty and randomness of a power grid operation mode and a climate state at the same time; obtaining power grid operation data through a power grid energy management system EMS; when the uncertainty of a power grid operation mode is considered, mainly introducing the uncertainty operation state of a line; when the uncertainty of the climate state is considered, mainly introducing the uncertainty state of the climate; and assuming that a load is a deterministic load, that is to say, there is no relationship with the probability of environment-dependent failure of the power transmission line, calculating the probability of environment-dependent failure when the line passes through regions with different weather conditions (assuming that the weather conditions of two adjacent regions are different) on the basis of probability analysis, and providing necessary technical support for determining the reliability of the power transmission line.