In unpowered trajectory of ballistic missiles, according to the role of air drag from its trajectory can be divided into free-flight trajectory and reentry trajectory in two parts. The paragraph [1] is simulation analysis for free-flight trajectory and the paragraph [2] is simulation analysis for reentry trajectory. In fact, the free-flight trajectory and reentry trajectory has close contacts, through these contacts, established unpowered trajectory of ballistic missiles applying to the entire trajectory equations. Finally, as an example of a certain model of short-range ballistic missiles to carry out the establishment of a simulation model, through simulation and analysis of results, the model can be a better reflection of the dynamic characteristics of ballistic missiles' unpowered trajectory.