This paper applies the agent technology and the natural language understanding technology into the question-answering system. Firstly, it introduces the agent technology and the natural language understanding technology; and then a new design method is proposed and system's model including the problem analysis agent model and corpus and search algorithm is constructed. It has solved the problem of computer understanding Chinese, as well as improved the search efficiency and accuracy ratio of question-answering system; at last these methods are applied into the domain of forestry pests and diseases and receive better effect.