Abstract: The in-plane parametric resonance instability of composite laminated circular arches under a vertical base excitation is investigated. The equilibrium differential equations of dynamic stability and the Mathieu-Hill equation of an arch are derived based on the Hamiltonian principle. The dynamic instability regions corresponding to period 2T and period T are obtained. The effects of rise-span ratios and ply-angle on the dynamic instability region are researched. It is found that the dynamic instability region where the parametric resonance mainly appears near twice the natural frequency of the structure. With a decrease of rise-to-span ratio, the bandwidth of the dynamic instability region increases, and the critical excitation frequency increases accordingly. Compared to the angle-ply laminated arch, the cross-ply arch shows a smaller bandwidth of the instability region and a larger critical excitation frequency. For an angle-ply laminated arch, the bandwidth of the dynamic instability region increases and the critical excitation frequency deceases with the increasing ply-angles.