Abstract: The ecological environment is closely related to human life. Rapid and effective evaluation of regional ecological environment quality and spatial-temporal changes is helpful to understand the status of regional ecological environment, and provide the basis of development trends for the regional ecological environment protection and sustainable development. In the past few years, remote sensing ecological index(RSEI)based on PCA has been increasingly more widely applied to ecological environment evaluation of different fields and achieving good results. In this paper, based on Landsat TM/OLI remote sensing data of 4 periods in 26 years(1989-2015), four ecological evaluation indicators including wetness, greenness, heat and dryness were retrieved, which were the important ecological indicators frequently used in assessing regional ecology. Four indicators were developed RSEI and ESI(ecological environment quality synthetical index)of different periods using PCA method. Yanchi County ecological environmental quality from 1989 to 2015 was evaluated and spatial-temporal evolution characteristics of ecological environmental were discussed based on RSEI and ESI. Firstly, using remote sensing images based on pre-treatment, four indicators was obtained from Tasseled Cap Transformation, including WET, NDVI (Normalized Difference Vegetation Index), LST(Land Surface Temperature)and NDSI(Normalized Difference Soil Index). Then these four remote sensing ecological indices were used to build the RSEI with PCA. Based on RSEI, five levels of ecological zones were identified using equal interval method and ESI was calculated. Finally, RSEI change detection of Yanchi County during different periods were classified to five levels and mapped. The results show:(1)The humidity degree and green degree have a positive effect on promoting the ecological environment quality of the region, while the heat degree and dry degree have a restraining effect on the regional ecological environment quality, and NDVI which represents green degree has the most contribution to ecological index.(2)The whole situation of ecological environment quality in Yanchi County has been improved apparently from 1989 to 2015, and the mean value of RSEI was 0.41, 0.54, 0.51, 0.57 in 1989, 1999, 2006, 2015 respectively, which indicated that in 1989, 1999 and 2006, the level of overall regional ecological status of Yanchi County was general level, but in 2015, it was good level. The ecological environmental quality of the study area showed the characteristics of rise-decline-rise, the overall ecological environment quality has been improved apparently. The area of significantly improvement of ecological environment quality accounted for 20.01% of total area; howerver, the area of significantly deterioration of ecological environment quality just accounted for 1.64% of total area during the 26 years. The improved regions mainly distributed in the area of Yanchi-Gaoshawo-Wanglejing which was in the north of the county, and Mahuangshan area which was in the south of the county, the deteriorated regions mainly distributed in the middle of Fengjigou-Dashuikeng.(3)The precipitation had significant impact on change of ecological environmental quality of the study area, and desertification control measures such as returning farmland to forests and pastures, forbidding grazing grassland and prohibiting deforestation played an important role in promoting the ecological environmental quality.

Key words: remote sensing, ecological environment quality, remote sensing ecological index(RSEI), Yanchi County