In the case of pier-group structures in offshore engineering construction,the calculation of wave forces on piers is quite complicated due to the wave scattering effect on the piers in group.In this paper,the diffraction problem of two piers with unequal diameters is studied and formulas for calculating wave forces on piers are given on the basis of MacCamy-Fuchs linear wave diffraction theory with a strict consideration on the phase difference between the piers in a pier-group.