The invention discloses a method for improving a displaying effect of a display screen and a terminal device thereof. The terminal device comprises an optical sensor, a main chip and a photoreceptive integrated circuit (IC). The method includes the following steps: a, a plurality of groups of display screen gamma values are set, and each group of gamma values correspond to light intensity of one ambient light range; b, The optical sensor senses ambient light, obtains light intensity, and sends the light intensity to the photoreceptive IC; c, the photoreceptive IC analyzes the ambient light intensity, obtains a tone value, a brightness value and a saturation value of the ambient light, and sends the tone value, the brightness value and the saturation value of the ambient light to the main chip; and d, the main chip calls a preset display screen gamma value according to the ambient light and sends the gamma value to the display screen, and the display screen adjusts contrast ratio. Under different ambient lights, corresponding display screen gamma values are set to adjust the contrast ratio of display screen, and the purpose of improvement of the displaying effect of the display screen is achieved.