In the output section of klystrons, microwave energy is extracted from kinetic charged particles through beam-wave interaction process. In this region, the electron velocity will suffer violent changes, and some electrons may become so slow that they are overtaken by other ones. The traditional time integral method cannot track the laggard particles effectively, so its computation result is doubtful in the case of serious surpassing phenomenon existing in electron beam. To solve this problem, we present a new dynamic track method(DTM) that can correctly calculate the space charge force from other particles on the lagged ones. We will briefly explain DTM's principles and introduce a one-dimensional program based on this idea. Further, we apply this new program in output-section calculation of klystrons. The results show better agreement with experimental data than the traditional method.