Based on the linear potential theory, an analytical solution is presented to study the diffraction problem of a hemisphere on the surface of water with infinite depth. The incident wave potential in the spherical coordinate system was proposed and the method of multipole expansion was employed to obtain the diffraction wave potential in the form of infinite series of the associated Legendre functions with the unknown coefficients which could be determined by solving a set of linear equations from the body boundary condition. The wave exciting force acting on the hemisphere was calculated and the results were compared with those from the Haskind relationship and some numerical results from the published literature.