The invention discloses an effective order estimator for a predistorter, which comprises a conversion unit, a decomposition unit and an approximation unit, wherein the conversion unit is used for receiving a polynomial of the predistorter, and converting the polynomial into matrix representation to obtain a predistortion matrix consisting of K predistortion functions, wherein K refers to the maximum order of the polynominal; the decomposition unit is used for performing singular value decomposition on the predistortion matrix to obtain a singular value matrix; and the approximation unit is used for acquiring the effective sequence of the predistortion matrix according to the singular value matrix, wherein the effective sequence of the predistortion matrix is the effective order of the polynominal and is the minimal sequence of an approximate predistortion matrix capable of achieving the preset approximation effect of approximating the predistortion matrix, and the approximate predistortion matrix is a matrix for approximating the predistortion matrix. The effective order estimator reduces the complexity of the polynominal-based power amplifier predistorter while ensuring the predistortion performance so as to have better convergence property and stability.