Ecological redline zone, as an important ecological function area, plays a fundamental role in guaranteeing the national ecological security. How to accurately and effectively assess the ecological quality evolution of ecological redline zone and the implementation effects is of great significance to further improvement of the "red line" policy and regional sustainable development. Based on the Google Earth Engine platform, Landsat 5 and 8 long-time sequence remote sensing images were used to establish a Remote Sensing Ecological Index (RSEI) for Jiangsu Province from 1985 to 2021, with which the quality change from 1985 in the ecological redline zone was analyzed and the role of ecological redline policy implemented since 2013 was assessed. The results showed that:(1) the average contribution rate of the RSEI model PC1 established for Jiangsu Province is higher than 70%, which applies to the analysis of the evolution of ecological quality in Jiangsu Province and the ecological redline zone. From 1985 to 2021, 58.2% of the total area of Jiangsu was improved in terms of RSEI, and the average RSEI value of the ecological redline zones varied between 0.52 and 0.63, showing an overall fluctuating upward trend. (2) The year of the sudden change in the mean value of RSEI in the ecological redline zones as a whole was 2003, and the sudden change in the mean value of RSEI in ecological redline zones in each city was mostly concentrated between 1995 and 2005, with some cities in southern Jiangsu and central Jiangsu showing a sudden change from good to poor in the 1990s, and cities in northern Jiangsu showing a sudden change from poor to good in the 2000s. (3) The ecological redline presented a shielding effect in blocking the encroachment of impervious surfaces and bare soil, such as building land, into the protected area, with a significantly higher trend of RSEI growth within the ecological redline zone than outside. Among the 13 cities, Nantong, Suzhou, Wuxi, Changzhou, and Zhenjiang were the cities with significant improvement, Nanjing, Xuzhou, and Taizhou were the cities with accelerated improvement, Lianyungang and Huai'an were the cities with slow improvement, Suqian, Yangzhou and Yancheng were the cities with insignificant improvement of RSEI, the "red line" policy has no obvious effect on improving the ecological quality of the ecological redline zones in these three cities. This method can be used for the operational monitoring and assessment of the implementation effects of ecological redline policy.