Abstract: In 2014, Nujiang Lisu Autonomous Prefecture (Nujiang Prefecture for short) in Yunnan Province officially launched a new round of converting cropland to forest project. In order to explore the impact of the round of conversion of cropland to forest project on the quality of the ecological environment in Lanping Bai Pumi Autonomous County (Lanping County for short) in Nujiang Prefecture, three periods of time were selected: before the implementation of the project (2013), during the implementation of the project (2017), and at the end of the project (2020). Based on remote sensing ecological index (RSEI) model, four indexes, including greenness, dryness, wetness and heat, were selected and principal component analysis method adopted to study the dynamic changes of the ecological environment quality in Lanping County from 2013 to 2020. The results showed that: 1) The ecological environment quality of Lanping County had been on the rise from 2013 to 2020, and the ecological environment quality of Zhongpai Township, Shideng Township and Hexi Township improved significantly. 2) From 2013 to 2020, the ecological environment quality level of more than 75% of the regions had remained unchanged, and the ecological environment quality levels of other regions changed slightly. The area of better ecological environment was larger than the area of worse ecological environment. 3) The improvement degree of the ecological environment quality of the converting cropland to forest patches was higher than that of 2 km buffer zone of the converting cropland to forest project and the whole study area. The conversion of cropland to forest project had a significant impact on the improvement of the ecological environment quality. The ecological environment of Lanping County could be improved by further promoting the project of converting cropland to forest. 4) The potential abandoned farmlands in the study area were mainly distributed in the four townships of Zhongpai Township, Shideng Township, Yingpan Township and Tu'e Township, which could be used as key areas for further conversion of farmland to forests.armland to forests.