The quantity of detritus changes monthly and there is a peak value in its changing curve. A model for litter-fall varying in mangrove forest can not he established by usual regression methods or fragmented fitting. The two questions to establish the model in common ways, one is about how to determine the transform point, the other how to make the curve continuous at the point, have been solved by a unit step function. The model of the unit step function based on a sign function was developed, and according to the model, the other litter-fall ingrated model with the peak value was created. The ...