Concerning the periodicity and time-varying characteristics of air traffic system operation， a flight conflict network situation prediction method based on Optimal Training Set Online Fuzzy-Least Squares Support Vector Machine （OTSOF-LSSVM） was proposed by combining complex network theory and fuzzy Least Squares Support Vector Machine （LSSVM）. Firstly， a flight conflict network model was constructed based on the three-dimensional velocity obstacle method， and conflicts were judged according to the positions， headings and velocities of the aircrafts. Then， the evolution time series of topology indicators of flight conflict network were analyzed to obtain the optimal training set which consisted of samples related to the predicted moment in time and distance. Finally， a prediction model was obtained by online fuzzy LSSVM training， and the idea of block matrix was used to simplify the updating process and improve the efficiency of the algorithm. Experimental results show that the proposed method can quickly and accurately predict the air situation， provide reference for controllers to master the development of air traffic， and assist the pre-deployment of conflicts.