We put forward a computational model of natural language in which the process of communicating in natural language is divided into three levels: linguistic form, surface semantics and deep semantics. Based on this model we design a Chinese language understanding system which could be easily transported from one domain to another . A new grammar, Chinese Semantic Construction Grammar, is introduced, which could describes not only syntactic form but also semantic form of a sentence and performs syntacric anallysis and semantic anslysis simultaneouslyduring parsing.The deep semantics of a word or a phrase and the basic oprations of deep semantics are formally defined.The algorithms of the parser,the parser,the understander and the generator of the system are also presented.