This paper presents a solution for the second-order diffraction potential of an arbitrary three-dimensional large structure fixed in a monochromatic wave. The second-order Green’s functions are derived in frequency domain, by using Fourier series and addition theorem of Bessel Functions as well as Hankel transformation. The use of the second-order Green’s functions makes the second-order velocity potential satisfy the non-homogeneous second-order free-surface conditions and radiation conditions exactly. The second-order Green’s functions are expressed in the form of conventional Cauchy PV integrals which appear to be easy to implement numerically. The second-order velocity potential is expressed as the sum of the set of particular solutions evaluated by using the second-order Green’s functions and the homogenous solution.