Abstract: The flutter characteristics of a twin-fuselage aircraft are studied. An unsteady aerodynamic model is established by using the double lattice method (DLM) and stipe modified double lattice method. Characteristics of structural dynamic was found using ground vibration test (GVT), based on which equivalent beam models with different fuselage stiffness was established. The influence of fuselage stiffness on structural dynamics is found by finite element method (FEM). The influence of fuselage stiffness on the flutter of twin-fuselage aircraft was investigated by solving the coupled frequency domain equation. The results show that the Z-direction stiffness of the fuselage cross-section has a great influence on fuselage 1st vertical symmetric bending mode, fuselage 1st vertical anti-symmetric bending mode and horizontal rolling mode. When the Z-direction stiffness of fuselage section reduced to the 67% of the original design, the torsion effect of the fuselage 1st vertical anti-symmetric bending mode on the horizontal tail enhanced, the twin-fuselage aircraft flutters at 269.34 m/s and flutter frequency is 37.2 Hz.