The invention provides a distributed transmission and distribution cooperative reactive power optimization method and system based on Thevenin equivalent parameter identification. A global optimization problem of a power transmission network and a power distribution network is decomposed into sub-problems that each master system and each slave system independently carry out reactive power optimization. For the power transmission network, each power distribution network connected with the power transmission network is simplified into a PQ load, each power distribution network only transmits a complex power value at a root node to the power transmission network, and the complex power is the sum of net load of the power distribution network and complex power loss of the power. For the distribution network, the power transmission network is equivalent to Thevenin equivalent potential and equivalent impedance. The relative independence of an original power transmission and distribution network system and data is maintained. The state of the whole power transmission network system is represented only through the two parameters of the equivalent potential and the equivalent impedance, theinformation transmitted to the distribution network is less, and the communication traffic is reduced.