A settlement of surface is an environmental hazard. A new model was proposed to forecast exactly the change law of a settlement during the complete process of construction and operation. Either Poisson curve model or Verhulst model is a special case of the new model. A new concept and method are proposed to break through limitations of a three-piece algorithm used by Poisson curve model. It combines a nonlinear regression method and a cubic spline interpolation method. Solutions to the new model are obtained by the new method. An example is given and discussed. Results show that the new concept and method can give precise solutions of the model and that the resolve between calculated results from the new model and measured ones is reduced greatly compared with that from Verhulst model. The new model can provide geotechnical engineering design with new scientific basis.