A dimming method and dimming device for a backlight module are provided. The dimming method includes the following steps. Light distribution information of a plurality of light sources of the backlight module corresponding to a display panel is provided. According to the light distribution information of each of the light sources, a plurality of light contribution ratios of the light sources corresponding to a plurality of different positions of the display panel are obtained respectively. According to the light contribution ratios corresponding to the different positions, the needed intensity of each of the light sources is determined respectively.