For the mechanism modeling method is difficult to predict the parameters of VAV system accurately, this paper use BP neural network to build a predictive model of VAV system and combine the genetic algorithm with BP network. So that it can overcome the disadvantages of BP algorithm such as low speed converge and easily being subject to the partial minimum, because of using genetic algorithm to search the weights and thresholds of BP network to find the optimal solution before BP operation carried out. After collecting a lot of data for training and verifying model through the experimental platform, the results show that the model predictions can simulate the measured data accurately on the temperature and humidity of the room and the parameters of air conditioning, and with a high generalization capability.