Abstract: According to the extrapolation property of data fitting curve and the orthogenality of the Legendre polynomial, Discrete Legendre Polynomial Series (DLPS for short) is developed in this paper. A series of properties and an real-time on-line iterative algorithm are proposed. The amount of calculation is much smaller than that of all kinds of original algorithm, and the problem of calculation time in forecasting control is solved. Experiments and applications in production floor show that the forecasting principle and algorithm proposed is powerful.