Abstract: For structural dynamic systems, the design-point excitation plays an important role in structural reliability assessment and random vibration. The calculation method of design-point excitation is still an important research focus today. This paper suggests a procedure to obtain the design-point excitation of non-linear systems based on mirror-image excitation of the free-vibration response. It is shown that the design-point excitation thus obtained is effective in estimating excursion probabilities in non-linear dynamic systems when compared with the Monte Carlo method. In order to further illustrate the effectiveness of the method, the paper gives a numerical example of linear dynamic systems. We obtain the design-point excitation based on both the analytical method and mirror-image excitation. It is shown that the two methods have the same effectiveness when estimating excursion probabilities.