A mobile terminal-based method is established for testing the influence of ambient light on outdoor readability and the relevance of device brightness and ambient light. Gray sinusoidal fringes of different frequencies and different brightness centers are observed under different ambient light and different device brightness. According to the recorded just-noticeable lightness threshold, the influence of ambient light on the just-noticeable lightness threshold is analyzed and the model of simplified ambient contrast and just-noticeable lightness threshold is established. The results show that the influence of ambient light is inversely proportional to the contrast sensitivity of the observed sinusoidal fringes. A quite specific function can be got by the model and it can be applied to the setting of the device brightness under different ambient light.