Bailongjiang River watershed (BRW), a typical geomorphic area with frequent-hazards, located in the transitional ecotone among the Loess Plateau, Qinba Mountains and Tibet Plateau, was chosen as the study area to analyze the landscape ecological security (LES). Based on the Pressure-State-Response (PSR) conceptual framework and platforms of GIS and RS technology, an evaluation system for landscape ecological security was suggested with three dimensions (Pressure, State and Response) from 1990 to 2010. The three components, namely each of the three dimensions and two factors specific to the particular dimension, were determined separately from each other, and then integrated into a single rating. The factors were also divided into two types, depending on their impact on the ecological security: positive (+) or negative (-). Here the pressure index, which reflected the pressure that geomorphic hazard exerted on the environment, consists of landslides, debris flows, soil and water erosion. The landscape state index, which was related to the quality of the environment, was composed of landscape vulnerability index and ecological service value. The response index, which was described as reaction of human beings to landscape dynamics, was defined as adjusted intensity of natural reserve districts, water area, forest, grazing district and crop lands. And then, the landscape ecological security was classified into 5 levels: low, slight, medium, medium-high and high. In the same time, the evaluation units were Landsat TM pixels and the fundamental data set was obtained by classifying raw TM data in this study; and the factors used in PSR model were obtained by using different data from meteorological station, soil surveys and maps, digitized topographic maps, NDVI, DEM and Landsat TM images. The results showed that: the maximum value of landscape pressure index was 0.697, 0.699, and 0.709 in 1990, 2002 and 2010 respectively, although the average value change was not obvious, which means the landscape pressure increased locally. The values of landscape state index and response index increased from 0.531 and 0.802 in 1990 to 0.550 and 0.815 in 2010, respectively, suggesting that landscape security became more secure. After then, landscape ecological security, which was the weighted sum of the pressure index, the landscape security state index and the response index, increased from 0.623 in 1990 to 0.636 in 2010. In the meantime, the change of LES pattern was small and trended to be better, and the cover area of medium to medium-high grades increased (2 089.95 km2) during the whole period of 1990-2010; especially after 2002, the proportion of above high ecological security levels was up to 52%, showing that the performing of ecological engineering (such as Grain for Green, Construction of Shelter Forest System in the Upper- Middle Changjiang River.) were very effective. As to the spatial distribution pattern, the districts with low ecological security mostly located along the Bailongjiang River and its tributaries, northwestern part of Tanchang County and northern mountains of Diebu County, while the high ecological security happened as nature reserve and forest district. And then, the distribution pattern of LES was opposite to that of landscape ecological risk on the landslides, debris flows, soil and water erosion. This research showed that the PSR model based on gridding GIS was a useful way for the quantitative evaluating and mapping LES under natural hazard stress and can be used in other similar regions.