Abstract: Cylinder is one the most common onshore or offshore marine installation.Aalculating appropriately the wave loads acting on the cylinder is important.The paper studies the calculaton of wave force on large scale cylinder.Using the linear wave diffraction theory to compute wave force on the large scale cylinder,the paper presents a Nyström method to solve the problem of the linear wave diffraction.The mathematical model for the linear wave diffraction is transformed into a two dimension Helmholtz equation by the airy wave theory.And the Nyström method is used to solve the Helmholtz equation.The numerical examples illustrate its accuracy.