An important aspect of achieving the "dual carbon" goal as scheduled is to reduce land use carbon emissions. Carrying out ecological restoration projects is an important starting point to reduce land use carbon emissions. To this end, China has continuously deepened the deployment of ecological restoration projects and implemented the ecological protection and restoration project for mountains-rivers-forests-farmlands-lakes-grasslands (MRFFLG) to cope with global climate change. As a comprehensive ecological restoration project, the MRFFLG project provides feasible ideas for carbon emission reduction by improving or restoring the structure and functions of existing ecosystems and optimizing the spatial layout of regional ecosystems. How to evaluate the impact of MRFFLG projects on land use carbon emissions has become a core issue. Based on panel data from 287 prefecture-level cities in China from 2010 to 2021, using the time-varying difference-in-difference method, the study focuses on the impact effect and mechanism of the MRFFLG project pilot policy on land use carbon emissions. The results show that (1) through a series of robustness tests such as parallel trend test, placebo test, sample data screening, exclusion of other policy interference, and PSM-DID test, the MRFFLG project pilot policy has a significant inhibitory effect on land use carbon emissions in pilot cities, reducing emissions by 29.9% when considering regional and temporal characteristics as well as other influencing factors. (2) In terms of mechanism of action, the MRFFLG project pilot policy can reduce land use carbon emissions through three paths: natural vegetation restoration, land structure optimization and green technology innovation. (3) In terms of urban heterogeneity, the MRFFLG project pilot policy is conducive to resource-based cities to actively get rid of the "resource curse" and achieve green transformation and development, thereby more effectively reducing land use carbon emissions. Similarly, the superior human, technological and resource conditions in the central and eastern regions can further strengthen the role of the MRFFLG project pilot policy in reducing land use carbon emissions. Although the MRFFLG project pilot policy promotes the improvement of the ecological environment, limited by the economic scale, innovation capabilities and geographical location of non-resource cities and the western region, the endogenous driving force for carbon emission reduction is insufficient, which affects the implementation effect of the MRFFLG project pilot policy. In the future, targeted policies should be closely followed by the three major mechanisms, urban types and location differences should be paid attention to, and the carbon emission reduction effect of the MRFFLG project should continue to be exerted. The research results are of great significance for improving the top-level design of the MRFFLG project and promoting the realization of the "dual carbon" goal.