The sub-impact phenomenon of a simply supported beam struck by a steel sphere were studied by the Hertz impact theory and the three-dimensional dynamic finite element method,and tested by a set of sub-impact experimental device self-designed and its corresponding transient measurement system of the sub-impact. The sub-impact phenomenon was thoroughly discussed and studied through the experimental tests and numerical simulations. The results show that:（ 1） the impact duration of the first sub-impact is coinciding very well with the Hertz impact duration;（ 2） The subsequence sub-impacts have some different characteristics from the first sub-impact. And their impact durations vary randomly around the Hertz impact duration,but their variation in general are coincide with the Hertz impact duration;（ 3） It shows that the complicated sub-impacts that look like random and sensitive to structural transient response are dominated by local impact-contact deformation,and their mechanism can be explained by the Hertz impact theory;（ 4） The theoretical analysis has shown that the processes of the subsequence subimpacts are influenced greatly by the global structure vibrations,in turn affecting sphere motion and beam vibration response,and eventually leading to the next collision is difficult to predict.