Abstract: The frequency of power system and the rotational speed of generator are the derivative of generator rotor angle,so they can be controlled by the generator rotor angle.This paper describes the mechanism that,after rotor angle droop control is implemented,generators can autonomously adjust the power output to share load variations and restore frequency without dispatching center intervention.Then the relationship between the power output variation and the rotor angle variation after the transient process（caused by load change）ends is provided.Further,the power flow calculation method that can be used after the implementation of rotor angle droop control is given.The analysis shows that,after slight modification,the NewtonRaphson method can also be used as a power flow calculation method under the rotor angle droop control.The calculation results can be well matched with the simulation results.The method can be used for static safety analysis or state estimation calculation after implementing the rotor angle droop control.