The boundaries of natural harbour or artifical harbour are very complex as far as its shape and the reflection condition are concerned. The schemes available for solving wave diffraction in a harbour are not very accurate due to the simplification of realistic boundaries into a single jetty, a gap breakwater and a detached breakwater, neglecting the reflection effect of boundaries along the quays and the shore line whose shapes are rather irregular. Physical models turn to be quite helpful and valid in this case. In the present paper a mathematical model for wave diffraction in a harbour is introduced by using the singularity distribution method which can take into account the boundaries with arbitrary rate of reflection. The present model has been applied to the estimation of wave diffraction in the harbour basin of Guangdong Dayawan Nuclear Plant, and verifications with corresponding physical models are quite favorable. The boundaries of natural harbor or artifical harbor is very complex as far as its shape and the reflection condition are concerned. The schemes available for solving wave diffraction in a harbor are not very accurate due to the simplification of realistic boundaries into a single jetty, a gap breakwater and a detached breakwater, neglecting the reflection effect of boundaries along the quays and the shore line whose shapes are rather irregular. Physical models turn to be quite helpful and valid in this case. In the present paper a mathematical model for wave diffraction in a harbor is introduced by using the singularity distribution method which can take into account the boundaries with arbitrary rate of reflection. The present model has been applied to the estimation of wave diffraction in the harbor basin of Guangdong Dayawan Nuclear Plant, and verifications with corresponding physical models are quite favorable.