Abstract: For testing the capability of short term conflict alerting of air traffic control system, two methods are usually used. The former is to set a higher threshold, use the real data testing whether the system can alert when distance between two flights gets lower than the threshold. However, this method is not reliable. The second method is simulating flights which will conflict and obtain their trajectory from calculating, and then send these data to ATC system to see its reaction. This method is usually too simple to test whether the system can pre-detect a conflict effectively. To solve these problems, a probabilistic approach is used in this paper to simulate aircrafts with given probability of conflicting. Firstly, we derived the conflict probability of turing flights from Prandaini’s method of conflict probability estimation for linear flight. Then using reverse derivation we got the motion parameters of two targets whose conflict probability was pre-setted. At last, we simulated this pair of targets’ track and anlysised their conflict probability. The simulation results show that the targets’ probability of conflict was in line with the previous assumption. The trajectories generated by this algorithm are more realistic then a more effective conclusion of ATC system’s capability of short term conflict alerting and pre-detecting will be provided.