Vegetation is crucial for wind-breaking and sand-fixing, and is an important basis for testing the ecological protection effect of wind-break and sand-fixation regions. The study area is the national key ecological function zones of wind-break and sand-fixation designated by the National Plan for Major Functional Zones of the State Council in 2010, as well as the county areas of transfer payment for national key ecological function zones. Based on the Moderate Resolution Imaging Spectroradiometer (MODIS) Leaf Area Index (LAI) datasets from 2000 to 2019, this paper analyzed the LAI trend of the study area in both ecological zone scale and pixel scale, and the impacts of climate factors on LAI were also analyzed. The above-mentioned works can reveal the state of ecosystem in ecological function zone for wind-break and sand-fixation, providing advices for ecological protection. The results show that, during 2000-2019, annual year average LAI decreases from east to west across the study areas, and an overall increasing exists as time went by, with a rate at 0.03 m² m^-2(10a)^-1 (P<0.01). At the ecological zone scale, the LAI of the eight ecological function zones increases with various amplitudes, and the increasing trend of LAI from 2010 to 2019 is obviously larger than that in 2000-2009. Among the eight ecological function zones, the Horqin Grassland Ecological Function Zone has the largest increasing amplitude (0.1154 m² m^-2(10a)^-1, P<0.01) over the last 20 years. At the pixel scale, the area with significant increase (P<0.05) in LAI accounts for 41.6% of the vegetated regions in the study area, among which 83.7% is covered by grassland and 11.2% is cropland. Pixels with increasing LAI mainly concentrate in the eastern part of the study area, showing a patchy distribution. LAI in the western part has a slight increase, distributing like a strip. Areas with increase in LAI account for 7.7% during 2010-2019, obviously larger than that in 2000-2009. The impacts of climate factors on vegetation in the study area are as follows:the increase of precipitation facilitates vegetation growth in the eastern parts, while in the western parts, these facilitating effects vanish, and the effect of temperature is weak across the whole study area. Apart from the impacts of climate factors, human interventions (implementation of wind-break and sand-fixation policy as well as the progress of agriculture technology) are also crucial on the improvement of vegetation growth. The LAI dynamics indicate that the ecological function for wind-break and sand-fixation in northern China has been improved.