The invention belongs to the technical field of mine filling slurry, and in particular relates to a filling slurry with improved fluidity and a preparation method thereof. The components of the filling slurry include fine-grained tailings, coal slime, modified carboxylic acid-based water reducer, alkali metal sulfate, alkaline activator, polypropylene fiber, bentonite, slag, gypsum, anionic surface active agent, cationic surfactant and water; by adding modified polycarboxylate superplasticizer, the sensitivity of the filling slurry can be reduced, the rheological properties of the slurry can be improved, and the fluidity of the filling body can be adjusted to promote the fluidity of the slurry and stability, as well as the use of anionic and cationic surfactants, etc., through surface charge and steric hindrance, to achieve drag reduction and high-concentration delivery, reduce bleeding rate, eliminate the risk of pipe plugging in the filling system, improve filling capacity, and achieve Stable and economical delivery of filling slurries.