Abstract: Grassland carbon sink plays a pivotal role in China’s efforts to achieve its "dual carbon" strategic goals. However, climate change and intensive human activities have jeopardized the realization of grassland’s carbon sequestration potential, creating an urgent demand for a robust policy guarantee system. While existing studies have conducted preliminary text sorting of grassland carbon sink policies, there is a notable research gap in the integrated analysis of the evolution trajectory and effectiveness evaluation of regional policies. Northwest China, with its vast grassland areas and fragile ecological environment, is a key region with substantial grassland carbon sink potential in China. Against this backdrop, this study took 189 grassland carbon sink policies at all administrative levels issued from 2010 to 2024 across the five provinces in Northwest China as research samples, and constructed a policy evolution theme identification model based on the Policy Modeling Consistency-Latent Dirichlet Allocation (PMC-LDA) method. Drawing on classical PMC evaluation indicators and the actual context of carbon sink policies in Northwest China, we further established a policy theme identification and evaluation system consisting of 9 first-level indicators and 39 second-level indicators, to systematically assess the structural integrity and phased implementation effectiveness of the sampled grassland carbon sink policies. The results demonstrated that the evolution of grassland carbon sink policies in Northwest China can be divided into three distinct stages. The core policy themes identified via LDA have expanded from the initial two dimensions of "economic compensation" and "ecological restoration" to five dimensions, namely "industrial coordination", "market mechanism", "scientific measurement", "monitoring and evaluation", and "technological innovation". The overall average PMC index of the sampled policies was 6.69, corresponding to the Good rating tier. The average PMC indices for the three evolutionary stages were 5.95, 6.09, and 7.26, respectively, indicating that the policy effectiveness has upgraded from the General tier to the Excellent tier. Currently, the sampled policies covered a relatively comprehensive range of policy tools, yet there was insufficient focus on cross-sectoral coordination, market-based mechanisms, and technological innovation, along with notable disparities in policy maturity across provinces. Based on the PMC-LDA analytical results, this study put forward targeted policy recommendations for Northwest China: to enhance the collaborative governance capacity for grassland carbon sink management, to activate market-based carbon sink trading mechanisms, and to formulate differentiated regional policies. These measures are expected to boost the improvement of regional carbon sink capacity, and also provide a scientific reference for the optimization of grassland carbon sink policies nationwide in China.