An instant correction method for an encoder includes the following steps. The motion of a device under test is sensed to obtain a first wave signal and a second wave signal. The first and second wave signals are sampled to generate N first digital signal values and N second digital signal values. N positioning positions are generated according to the N first and second digital signal values, and the N positioning positions are added to a calculation group. A regression analysis is performed for the calculation group to generate a regression curve. The (N+1)-th prediction position is predicted using the regression curve. The ideal position of the device under test is determined at a time point of the (N+1)-th prediction position according to an ideal position curve. An error value between the (N+1)-th prediction position and the ideal position is applied to correct the device under test.