The ecological protection and high-quality development of the Yellow River Basin raise urgent demands for enhanced soil and water conservation strategies. Sediment water retention capacity is recognized as a critical factor influencing vegetation restoration and ecological management. Biochar is widely acknowledged for its dual potential in soil improvement and carbon sequestration. To address regional challenges, a systematic investigation was conducted to evaluate the effects of biochar type and grain size on water-retention properties of Yellow River sediment. Seven plant-derived biochar types (bamboo, rice husk, walnut shell, wood, corn stalk, corn cob, reed) were selected, and six grain sizes of bamboo-derived biochar (0.05, 0.18, 0.5 ~ 1, 1 ~ 3, 3 ~ 5, 5 ~8 mm) were systematically tested. Composite mixtures were prepared by incorporating biochar into sediment. Water-retention characteristics were measured, and microstructural properties were analyzed through scanning electron microscopy. Results reveal that biochar grain size exerts a significant control over sediment water-holding capacity, with a critical threshold identified to maximize retention efficiency. Biochar type demonstrates a particle-size-dependent influence on water retention, where bamboo-derived biochar achieves optimal performance. Microstructure experiments show that the addition of biochar optimized the pore size distribution of soil, especially with a significant increase in the number of water holding pores within 40 μm, enhancing soil moisture adsorption capacity. However, when the particle size of biochar is too small, it is easy to form agglomeration structures in the soil, completely filling some micropores and limiting the further improvement of water holding capacity. The research results provide a theoretical basis for achieving carbon sequestration and new utilization of regional resources.