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 detail gain lookup table and then looking up a corresponding gain adjustment according to each pixel in the primary image layer, so as to generate a 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.