Three mathematical models, i. e. regressive analysts method, artificial neutral method and fuzzy regressive method, are commonly used in the flood forecasting for special values. The practical calculation results of three cases provided in this paper show that either one is suitable for all cases. The regressive analysis method is favorable when the system has better linear correlations; otherwise the artificial neutral net method is better if the system is not linearly correlated. None of the above-mentioned method is reliable when the data needed for the calibration are not enough.