A method for adjusting an image signal by a processor includes providing a first low pass filter and a second low pass filter, generating an energy ratio of a band-pass image signal and the image signal according to the standard deviation of a first low pass signal of the first low pass filter and a second low pass signal of the second low pass filter, providing an image adjustment parameter and generating a weighting coefficient of the image signal according to the energy ratio and the image adjustment parameter, generating a third low pass filter according to the weighting coefficient of the image signal, the image adjustment parameter, the first low pass signal of the first low pass filter and the second low pass signal of the second low pass filter, and adjusting the image signal according to the image signal and the third low pass filter.