Coastal areas have become the focal regions of ecological environment changes during rapid urbanization and economic development. However, in coastal areas, the cloudy and rainy weather is frequent, leading quite limited remote sensing images with high quality. And due to variant dates of images, the prevalent Remote Sensing based Ecological Index (RSEI) from different periods can be hardly comparable. To solve this problem, we firstly synthesized Landsat images at any time using the Continuous Change Detection and Classification (CCDC) algorithm; afterwards, we reconstructed parts of remote sensing indices and modified the index normalization mode using time-series methods; and based on which, we finally developed a dynamic RSEI (DRSEI) and applied it on the coastal city of Ningbo to monitor the spatio-temporal changes of ecological quality as a case study. The results demonstrated that:(1) the RSEI was sensitive to timespan-the mean absolute deviation (MAD) of the RSEI reached 0.147 when the acquisition date of two real Landsat images was more than 1 month. This sensitivity would affect the stability and accuracy of long-term ecological quality monitoring. (2) In contrast, the MAD of the DRSEI from synthetic images based on CCDC algorithm was minor (0.097), relatively equivalent to the value between the two real images with timespan of half-a-month (0.072). That means, the sensitivity of RSEI to the timespan of real images were offset by the DRSEI using synthetic images to a certain extent. (3) The DRSEI was capable of monitoring regional ecological quality at any time. As an example, the annual (1986-2019) and semi-monthly (2019) DRSEI revealed the overall declining trend with increasingly spatio-temporal heterogeneity of the ecological quality in Ningbo City. To be specific, the rising DRSEI in forest areas and the falling DRSEI from the urbanization were two aspects for the spatial heterogeneity of ecological quality-the gradual increase of DRSEI in the southern and western forest areas could be seen in Ningbo City during 1986-2019 while the sudden decrease of DRSEI usually came from the conversion from farmland to built-up areas in suburbs during the urbanization. The DRSEI proposed by our study can accurately describe the overall trend and locate the turning points of regional ecological quality. Therefore, it can be used to correspond different stages and protection strategies during resource development, which is expected to serve the regularly ecological quality monitoring and assessment, supporting the high-quality development and ecological conservation for coastal areas.