Abstract: Cellulose nanofibrils (CNFs) show great potential for enhancing cement-based materials, but their susceptibility to degradation in the high-alkaline pore solution of cement may lead to the reduced long-term reforcement effect. This study aimed to improve the long-term reforcement effect of CNFs in cement matrix by developing a calcium carbonate-coated carboxylated cellulose hybrid material (CaCO₃@CNFs) based on a dual-modification strategy. Using bleached bamboo pulp as raw material, carboxylated CNFs were first prepared via TEMPO-mediated oxidation, followed by in-situ mineralization to form a CaCO₃ coating on the fiber surface. The results show that under optimized conditions, CNFs with a carboxyl content of 1.19 mmol/g were obtained, and a stable "core-shell" hybrid structure was successfully constructed by controlling the Na₂CO₃ solution dropping rate at a slow 0.5 mL/min. Cement paste incorporated with 0.10% CaCO₃@CNFs achieved 28-day and 90-day compressive strengths of 73.4 MPa and 85.5 MPa, respectively, with a 16.5% strength increase between 28 and 90 days, significantly outperforming pastes with only CNFs or nano-calcium carbonate. The CaCO₃ coating effectively protected the CNFs, enabling them to maintain structural stability and toughening function in the alkaline cement environment over the long term, while leveraging the early-age nucleation effect of nano-CaCO₃ and the long-term bridging and toughening effect of cellulose. This work provides a novel strategy for preparing high-performance and durable cement-based composites.