By taking into account the interactions between SPM and GVD,we obtained an analytic expression of chirp induced by both blue remove chirp and red remove chirp.the results show that the feature of chirp according to z and T is identified completely for certain L D and L NL .With different ratio of L D/L NL ,the feature of chirp have obvious changes.We found that unchirp propagation can be obtained by controlling the input power of pulse in fiber and the stability of pulse also can be maintained.