The relation between the chirp in phase of soliton pulse and soliton propagation is discussed in this paper. It is shown with numberical results that the chirp always changes sign whenever soliton pulse alternates between narrowing and broadening. The critical length of soliton propagation (with which the lumped amplifiers are separated) can be determined by means of the chirp, and the effect from this method is better than that from pulse width recovery scheme.