In this paper, two practical large cylindrical floating-roof tanks with diameters of 60 m and 100 m are selected as research objects. Wind pressures on the internal wall of tank shells under various liquid levels are carefully investigated by both experimental and numerical methods, especially those around the sealing device. It is found that computational fluid dynamics (CFD) simulation gives similar but generally greater results than wind tunnel tests (WTTs), and that wind pressures around the sealing device increase with the rise of liquid level. Geometrically nonlinear analyses (GNA) are then performed to figure out the variation of buckling behavior of tank shells with the liquid level. Two buckling modes are specified, featured by large buckles in the unstiffened region and local short-wave buckles in the stiffened region, respectively. Buckling loads are found to increase stepwise with the liquid level, with critical heights roughly calculated by positions of stiffening rings. Finally, recommendations on liquid levels maintained in strong wind conditions are given, considering the safety of both sealing devices and tank shells.