Abstract: Based on Bevington method, nonlinear full spectrum refinement of free decay curve was realized. Stability of the high resolution internal friction measurement under free decay mode of multi-function internal friction apparatus obtained by using the method can be up to ±7×10-6, which is almost one order of magnitude higher than that of linear amplitude fitting method. Especially, the better precision can be achieved via full spectrum fitting method in the case of high measurement damping.