According to the Liouville-von Neumann equation and WBR theory, a general description of relaxation of multi-spin system in the presence of an RF field is given. The relaxation equations are deduced and the theoretical computational formulas of various relaxation rates are obtained. It is almost impossible to accessthe analytical solutions of relaxation equations, so the computer program for numerical solutions is developed. By means of the program and the Bloch equations, the steady state solutions of two-spin system under various conditions are computed and these solutions are briefly discussed.