To solve the problem of slow convergence speed of BP network, a method is proposed to improve BP network by combining ameliorating learning rate and adding momentum factor. This paper developed a prototype system with improved BP network as the core of hot topics mining by Java programming language. Experimental results show that the system could acquire the classification results of hot topics according with the real public opinions quickly and exactly. The research result can help inform residents with instant hot spots as well as track public opinions so as to respond accordingly for governments and public enterprises.