In order to solve the propagation problem in the public opinion's spread process in the dynamic and uncertain Web community, we proposal a model which combines the gray theory differential equation model with the extended BP neural network model based on the analysis of gray theory system. Firstly, according to the network structure and propagation characteristics, the differential equation model of gray theory is established. Then, the model was mapped to the extended BP neural network. Finally, through training the neural network, the network model owns an ability of propagating prediction. Simulation results show that the combined model has a higher accuracy ratio than the single GM model in the propagation prediction of the Web community of public opinion's spread.