In order to improve the transient stability of the coordinated control system of generator excitation and static synchronous compensator (STATCOM), an adaptive backstepping robust coordinated control strategy based on immersive and invariant (I&I) adaptive backstepping control and L2‐gain control is proposed with consideration of the multiple uncertain parameters and external disturbance in the system modeling. The parameter adaptive law and coordination of control law are designed independently. The parameter adaptive laws for the two uncertain parameters: damping coefficient and mechanical power in the system are designed simultaneously by adopting I&I adaptive control method; the L2‐gain control is introduced in the backstepping design to eliminate the effects of the external disturbance on the system transient stability. The results of simulation show that the proposed strategy has strong adaptability to parameter fluctuations and robustness to external disturbances. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.