1. A tone adjustment method for a digital image, applicable to reduce a contrast deterioration degree of the digital image caused by a tone adjustment performed on the digital image, comprising: loading a digital image; generating a detail image layer and a primary image layer according to the digital image; performing a tone mapping procedure on the primary image layer, so as to generate a tone mapping layer; loading a positive detail gain table and a negative detail gain table; providing a selection window, so as to select a portion of image area in the primary image layer of the digital image; performing a standardized procedure on the image area of the selection window, so as to generate a representative pixel; when the representative pixel is positive, using the positive detail gain table for the positive representation pixel value in the detail image layer, looking up a corresponding gain value in the positive detail gain table according to the representative pixel, and then perform a gain adjustment on each pixel in the selection window, so as to generate a detail gain adjustment layer; when the representative pixel is negative, using the negative detail gain table for the negative representation pixel value in the detail image layer, looking up the corresponding gain value in the negative detail gain table according to the representative pixel, and then perform the gain adjustment on each pixel in the selection window, so as to generate the detail gain adjustment layer; performing a detail gain control procedure and then performing a detail gain adjustment on the detail gain adjustment layer and the detail image layer, so as to generate a gain correction layer; and combining the gain correction layer and the tone mapping layer and then outputting a combined layer.