Grayscale difference sensitivity reflects visual characteristics of human eyes, and people often watch images and videos on LCD screen now, so the research of grayscale difference sensitivity on LCD (liquid crystal display) is interesting and meaningful. Up to now, the experiment on sensitivity towards brightness change for human eyes is mainly divided into two categories: one focuses on rods or the other specific structures of retina, which requires high precision in controlling illumination devices; the other just selects some contrasts in different spatial resolution, and the experiment regards the eye as a whole instead of the specific structures. After analyzing advantages and disadvantages of the two kinds of experiments, we propose an interesting experiment based on Weber law to measure the grayscale difference sensitivity on LCD screen, which is helpful to distinguish the selected change and the region of interest. And under the actual illumination, the experiment is conducted on all gray levels. The experiment process are as follows: firstly, present an image in a certain gray range as the background on the LCD screen; secondly, select an area as the foreground randomly; thirdly, the tester adjusts foreground grayscale gradually until he can perceive the difference, and then the foreground position is marked clearly to verify the result given by tester; finally, the background and foreground grayscale are recorded simultaneously. The experiment is conducted under indoor illumination, 100 student volunteers who have normal or corrected visual acuity attend the test. To verify the experimental results, an image grayscale compression algorithm is proposed. The experimental results show that the distribution of grayscale difference sensitivity data is regular, and the experiment conforms to Weber law.