Regional ecological environments have been frequently affected by expansion of built-up land, which inhibits regional sustainable development. Therefore, it is of great significance to timely and precisely monitor regional ecological quality using remote sensing techniques. Using the Gui'an Development Zone (GDZ) in the Aojiang River Watershed as a case, this study investigated the regional ecological quality changes due to increased construction project-induced built-up land. Two rule-based algorithms were developed to obtain built-up land information from the Advanced Land Observing Satellite (ALOS) image in 2010 (before construction started) and the Gaofen-1 (GF-1) image in 2016 (after the construction). The remote sensing based ecological index (RSEI) was further employed to evaluate the ecological quality changes of the study area before and after the construction. As there are only four bands ranging from visible-near infrared wavelengths in the ALOS and GF-1 images, it is difficult to obtain the wetness and dryness components, which are needed for computing RSEI scores. To meet this requirement, the models of the wetness components specifically for ALOS and GF-1 images were developed. Two synchronous image pairs of GF-1 and Landsat 8 Operational Land Imager (OLI) were used to derive the coefficient of the wetness component of GF-1 by relating GF-1 data with the Landsat 8 wetness component based on selected pixel samples (i.e., 40384 samples). In addition, a soil salinity index was introduced to represent the component. The results showed that the area of built-up land of the study area increased notably over the six study years. The increase in the GDZ-induced built-up land represented 86% of the total increased built-up land in the study area. The overall ecological quality of the study area showed a trend of decline associated with the expansion of the built-up land in GDZ. The mean RSEI declined from 0.787 in 2010 to 0.689 in 2016, representing a 12.4% drop over the six study years. The area proportion of high ecological-quality grades also significantly declined from 91% in 2010 to 79% in 2016. Obviously, built-up land has a negative effect on ecological quality. Indeed, quantitative analysis indicated that a 10% increment in built-up land can cause a decline of RSEI by 0.041. Therefore, we suggest that green planning and development should be taken into consideration by regional planners during the forthcoming construction practice in the Aojiang River Watershed.