Abstract: The self-healing technology of cement-based materials is an important field of research for the timely and effective repair of engineering materials. This study used nano-micro silica fume to compositely modify hydrogels to enhance their crack self-healing ability. We investigated the influence of crack widths, types of hydrogels, hydrogel dosages, and initial particle sizes on the crack healing effect using the gas permeation method. Results show that (1) nano-micro silica fume significantly improved the water absorptivity of SAP, with a highest increase of 189 %. This enabled the cracked samples to have a lower gas permeability coefficient, especially under 0.3 mm wide cracks, showing a more explicit healing effect; (2) For 0.1 mm narrow cracks, hydrogels already exhibited good crack healing effects at a low dosage of 1 %, while for 0.3 mm wide cracks, hydrogels only demonstrated a higher healing efficiency at a high dosage of 3 %; (3) Either under wide crack or narrow crack conditions, the gas permeability of cracked samples containing larger initial particle size hydrogels was significantly lower than that of smaller particle size samples, indicating that hydrogel with larger particle size has better crack healing ability.