The invention provides a re-drying machine, and a water control fluctuation period acquisition method and system. The water control fluctuation period acquisition method can effectively acquire an autocorrelation coefficient and a partial autocorrelation coefficient of a time series X of a near-infrared water value, can calculate a large fluctuation period T1 of water control according to the autocorrelation coefficient, can calculate a small fluctuation period set {T2} of water control according to the partial autocorrelation coefficient, can intelligent perform corresponding adjustment on acontrol parameter of water control according to the large fluctuation period T1 and the small fluctuation period set {T2}, can stably control water in a re-drying machine, and is good in re-drying uniformity.