An apparatus and a method for processing signal are disclosed. The apparatus comprises an input interface and a processing unit. The input interface receives smoothing parameters and a signal to be separated. The processing unit establishes an upper extreme envelope and a lower extreme envelope, and calculates a mean envelope between the upper extreme envelope and the lower extreme envelope. The processing unit smoothes the mean envelope to generate a smoothed mean envelope according to smoothing parameters, and determines a trend component or a non-trend component according to the smoothed mean envelope.
