As the internet becomes more and more popular, a variety of bad information appears on the network such as eroticism, reaction, violence, etc. which seriously disrupts the order of the internet. The text information with bad contents diversifies on the internet. Targeting at the features changes of the emerged sensitive words included in bad text information, this paper puts forward a text pre-processing plan for filtering bad text information and a kind of filter system. In addition, it emphasizes the discussion on the methods for identifying and disposing of sensitive information arisen from the structure change of the sensitive words. At last, the paper presents a three-level text information filter model.