The effect of reactive power compensation largely depends on the reactive power compensation point selection. From the dynamic analysis methods for non-analytic complex variable power system, the impedance modulus margin index is pointed out to be the most intuitive indicator for voltage stability analysis, and it is used to preliminarily determinate weak line in system. Then this paper analyzes the system instable process under the transient disturbances, and points out that the transient stability margin is the essential indicator describing the stability of the system transient disturbance process, which can be quickly calculated by using extended equal area criterion. On the basis, a kind of dynamic reactive power optimization configuration method based on transient stability margin index is put forward, then the transient stability margin index increase values of various reactive power allocation schemes are compared to get the optimal one. By IEEE 39-bus system and the Guangdong Power Grid 220 kV network system examples, this method is proved to be effective.