In this paper, the theoretical model proposed in Zhang et al. (2019) is extended to compute the natural frequencies and modal shapes for two-dimensional moonpools with one or two recesses in finite water depth. In the framework of linear potential flow theory, the boundary value problem is solved by using a domain decomposition method, assuming the velocity potential at the outer boundaries to be nil. In particular, a new and efficient approximation method, double-mode approximations (DMA), is derived to estimate the natural frequencies and modal shapes for both piston-mode and sloshing-mode resonances. The present results using the derived DMA formulas are compared with the experimental results by Ravinthrakumar et al. (2019) and the solutions using the frozen-mode approximation (FMA). The comparisons show satisfactory agreement with the experimental data. In particular, it is shown that, in contrast to FMA, DMA can well predict the non-flat modal shape of the free surface at piston-mode resonance. Moreover, extensive parametric studies are performed to examine the effects of the moonpool geometry on natural frequencies and free-surface modal shapes. •The present paper studies how the moonpool configurations affect the natural frequencies and modal shapes of two-dimensional moonpools with one or two recesses.•Double-mode approximation (DMA) is derived to predict the natural frequencies and modal shapes of the piston as well as the sloshing modes.•The effects of moonpool geometry on the piston-mode frequency is revealed by analyzing the variations of the stiffness term and inertial terms in the frozen mode approximation (FMA).•The different parametric sensitivities of the piston mode and sloshing modes for the moonpool with recesses are discussed.