Neutron scattering experiments were performed to investigate magnetic order and magnetic excitations in ternary iron chalcogenide K2Fe4Se5. The formation of a superlattice structure below 580 K together with the decoupling between the Fe-vacancy order-disorder transition and the antiferromagnetic order transition appears to be a common feature in the A2Fe4Se5 family. The study of spin dynamics of K2Fe4Se5 reveals two distinct energy gaps at the magnetic Brillouin zone center, which indicates the presence of magnetic anisotropy and the decrease of local symmetry due to electronic and orbital anisotropy. The low-energy spin wave excitations of K2Fe4Se5 can be properly described by linear spin wave theory within a Heisenberg model. Compared to iron pnictides, K2Fe4Se5 exhibits a more two-dimensional magnetism as characterized by large differences not only between out-of-plane and in-plane spin wave velocities, but also between out-of-plane and in-plane exchange interactions.