The present disclosure provides a brightness regulating method, a regulating system and a displaying device. The method is applied to a displaying device. The displaying device includes a backlight module. The regulating method includes acquiring maximum grayscale values of the light areas and a maximum grayscale value of the backlight source; according to the maximum grayscale values of the light areas and the maximum grayscale value of the backlight source, obtaining a plurality of first brightness parameters; according to the plurality of first brightness parameters, obtaining a second brightness parameter; and subsequently, according to the second brightness parameter and the initial brightnesses of the light areas, regulating brightnesses of the light areas.