The technologies of reverse filtering, backward integral and energy level average are used to build the damping test data processing system for decay method; the relationship between the decay method test results and frequency-band damping in the sense of statistical energy analysis and the effect of modes outside the frequency band on damping test results are studied; in the end of this paper, a plate’s transient impact response data from finite element simulation are used to verify the relevant conclusions. The main conclusions include: if the difference of the modal damping of the modes in the frequency-band is small, the decay method results are close to frequency-band damping in the sense of statistical energy analysis while on the contrary the decay method results are less than frequency-band damping in the sense of statistical energy analysis; if there are modes with small damping outside the considered frequency-band, the damping test results will be strongly influenced by the mode; if the damping of each mode gently changes with frequency, the decay method test results are accurate, while on the contrary a large error will be caused.