This paper describes a Chinese keyboard intelligent fullsentence input method system based on tri-gram language model. In this system, we use efficient algorithms to reduce the size of the language model, accelerate the search and enhance the accuracy. The n-gram model is presented in a novel structure, which shrinks the model and enables it with look-ahead and buffer techniques to reduce the times of visiting the disk to fetch the n-gram unit and to adapt the language model to user domain quickly. Besides that, we have designed an efficient dynamic programming algorithm to segment input alphabetic sequence into syllabic cells; thereby it can be fit for different input ways.