Taking the towline respectively as "rigidized-flexible suspended cable " model and "linear spring" model, this paper builds the dynamic model of a towed air-launching system, simulates and calculates its full flight course in a longitudinal plane, including take-off, climb, level off and level flight. The simulation results obtained by applying the two towline models are almost the same, and can be used for assessing the feasibility of towed air-launch Reusable Launch Vehicle (RLV). And the algorithm for simulating the system flight is feasible. In addition, the towline's forces that act on the towed aircraft and the towed vehicle are studied in detail. The results verify mutually the feasibility of the two models of the towline. In a word, these two models are applicable for such multibody system.