The relevant calculation formulas of the steady force and the second-order force transfer function are derived from the second-order force calculation of the first-order potential. The second-order differential frequency forces of the incident and diffractive waves arethen approximately calculatedand the corresponding calculation program is compiled based on the method of AQWA. The calculated steady force and the second-order force quadratic transfer function of a floating production storage and offloading (FPSO) unit are compared with those results from the AQWA. It also analyzes the influence of the extended boundary integral equation method on the steady forces, and the contribution of the steady force components, such as thevariation of the wet surface and the square of the velocity potential gradient, to the resultants, as well as the variation of the quadratic transfer function of the second-order frequency difference force with water depth.At last, the problems that should be noticed are summarized.