Abstract: Abstract：Objective Compared with aircraft, wing-in-ground-effect (WIG) craft utilize the ground effect lift augmentation principle, which not only saves fuel but also offers advantages of low cost and enhanced safety. However, WIG craft suffer from poor longitudinal dynamic stability, posing a severe challenge to their design. For this reason, a biplane configuration (wing-tail layout) is commonly adopted to improve longitudinal dynamic stability. Determining the stability range is critical to the design process. Method First, the finite volume method and the SST k-ω turbulence model are employed to solve the Reynolds-averaged Navier–Stokes (RANS) equations, investigating the aerodynamic performance and longitudinal static stability of a single wing with the NACA4412 airfoil. The influence of the longitudinal center-of-gravity (CG) position on static stability is explored using the pivot-point theory. Subsequently, the dynamic stability of the biplane WIG craft is studied, and the mean logarithmic decrement is used to evaluate its dynamic stability. The mechanisms of various influencing factors are analyzed through flow-field visualization and other methods. Results The results show that the numerically simulated lift coefficient (CL) and drag coefficient (CD) are in good agreement with experimental data. Moving the center of gravity upstream benefits the static stability of the single wing, but the static stability criterion (3) cannot be satisfied within the range of 0.1C to 0.5C flying height (where C denotes the chord length of the front wing) and 2° to 10° angle of attack. Under certain conditions, the WIG craft exhibits dynamic stability when the front-wing angle of attack is 6° to 10° and the relative flying height of the front wing is 0.1 to 0.15. Moreover, the selection of the longitudinal CG position, the longitudinal tip gap between the front and rear wings, and the tail-wing angle of attack is constrained by both moment equilibrium and longitudinal dynamic stability. Conclusion The longitudinal dynamic stability range of biplane WIG craft using the NACA4412 airfoil as the front wing is relatively narrow, and such craft lack altitude-maneuvering capability. The longitudinal dynamic stability of biplane WIG craft is affected by multiple factors, and the requirements for achieving longitudinal dynamic stability often conflict with those for moment equilibrium across many parameters.