Abstract: In this paper, the influence of the preload on the dynamic characteristics of a system with screw connections among different components is investigated with the harmonic spectrum estimation method. It is shown that with a preload of the same order of magnitude, the system exhibits different dynamic characteristics or FRFs under excitations of different orders of magnitude, as a non-linear time-invariant system. High level excitation may invalidate the preload, thus lead to reductions of the stiffness, the natural frequency, and the amplification in the resonance region.