Based on the limited solution of three-dimensional (3D) Poisson equation, the dynamical properties of finite-thickness galactic disk is studied first. Calculation shows that the thickness effect cannot be neglected. It is found that the more realistic Galaxy model needs only about half of the halo masses than that with an infinitesimal thin disk. For a steeply decreasing halo density model, the thickness effect is more obvious. In addition, the dynamical properties of disturbed disk are investigated. By using the solution of 3D Poisson equation with Lin-Shu self-maintained density wave theory, a dispersion relation of 3D-density wave is set up. The local stability of disk is discussed, and the Toomre's local stability criterion is extended to the more realistic galactic model. Then we study the form of spiral arm, and show that only two armed spiral galaxies can be self-consistent with the logarithmic density disturbance, which is consistent with the observation. The group velocity of density waves is discussed, and it is shown that the group velocity problem can be soften by introducing the effect of thickness to a certain extent. Anyhow, it is shown that the finite thickness effect is a very important parameter in density waves theory. At the same time we put forward a tentative method to determine the thickness of galaxies.