The coaxial twin-rotor helicopter is one of the important directions for the future development of high-speed helicopters. To address the problem of large low-frequency vibrations of the fuselage caused by the dissimilarity of the two rotor blades in strong coupling, a dynamic balance adjustment method for suppressing the low-order harmonic loads of the upper and lower rotor hub harmonics was proposed. Firstly, a full-vehicle aeroelastic coupling dynamic model was established based on the Hamilton principle, and the model reliability was verified by combining with experimental data. Secondly, the low-frequency vibration of the fuselage was simulated by setting the rotor blade parameters to simulate the dissimilarity of the rotor blades in engineering, and then the first-order harmonic loads of the upper and lower rotor hub were set as the control targets, the counterweight mass applied to the rotor hub arms was the control input. Considering the constraints of the rotor cone, two sets of adaptive added-point surrogate models were constructed, and a cyclic iterative process with the fixed counterweight scheme was adopted to approximately simulate the global coupling. Finally, the optimized rotor hub arm counterweight scheme was obtained, which reduced the first-order dimensionless loads of the upper and lower rotor hubs by 81.5% and 82.2% respectively under the rated rotor speed operation, effectively controlling the lateral and vertical low-frequency vibrations of the fuselage of the helicopter in a stable level flight state. This method can provide a fundamental solution for the dynamic balance of the coaxial twin-rotor configuration helicopter, improving the flight safety and comfort of the helicopter.