Abstract: This study presents a robust adaptive control strategy which incorporates the Dynamic Equilibrium State (DES) theory to carry out balance control for underactuated acrobots with parameter variations and external disturbances. First, based on the idea of the stable state control of the DES theory, the optimal DES reference trajectories are planed for the angles of the links. Then, by combining the incremental sliding mode and fuzzy logic systems, a direct Adaptive Fuzzy Sliding Mode Controller (AFSMC) is designed to track the desired trajectories. In addition to the stability analysis, the robust performance of the proposed AFSMC against system uncertainties is verified via numerical simulations. At last, compared with sliding mode and fuzzy control methods, the simulation results show that the proposed method has stronger robustness and adaptive capacity to uncertainties of system parameters and external disturbances.