An image adjusting method of a display apparatus includes steps as follows: A plurality of preset grayscale-brightness relationship curves are determined corresponding to a plurality of time periods of a day, respectively. A reference grayscale-brightness relationship curve is selected from the preset grayscale-brightness relationship curves according to an acquisition time of an image data, corresponding to one of the time periods. A real-time ambient light is acquired, and it is compared with at least one of a reference contrast and a reference ambient light brightness corresponding to the reference grayscale-brightness relationship curve. At least one other grayscale-brightness relationship curve is selected from the preset grayscale-brightness relationship curves according to a comparison result to at least partially replace the reference grayscale-brightness relationship curve for adjusting the image data. The adjusted image data is then displayed.