The invention discloses a self-adaptive dynamic equivalence method for transient rotor angle stability online analysis of a power system. The invention belongs to the power system and automation technique fields. The method comprises the following steps of: calculating a detail degree index of dynamic equivalence carried out on the power system according to a transient rotor angle stability mode of the power grid after a certain expected failure under the power grid operation condition of the previous round, and the change of the current power grid operation condition relative to the power grid operation state of the previous round; carrying out the coherent equivalence on the generator of the rest generator group, the participation factor of which is less than the dynamic equivalence index, in the transient rotor angle stability mode of the failure; remaining nodes from which the electrical distances to other generators and the failure element are less than a set value and the branches directly connected with other branches; carrying out the corresponding network simplification on the power grid; and carrying out the transient rotor angle stability online analysis of the failure of the current round according to the equivalent power grid. Therefore, the calculation speed is improved by the invention.