The invention provides a composite superfine cement grout slurry and a preparation method thereof. The slurry is prepared by conducting ball milling on a matrix component and an additive component to obtain an ultrafine composite grouting material and mixing the water and cement according to a ratio of 0.5-0.8. The matrix consists of ordinary silicate cement, fly ash, slag and water; and the additive component comprises a grinding agent, an activator, a composite superplasticizer, an expansive agent and a retarder. The composite superfine cement grout slurry contains the following components by weight: 10% of slag, 0.8%-1.2% of composite superplasticizer, 5%-10% of expansive agent and 0.1%-0.6 of the retarder. The composite superfine cement grouting slurry provided by the invention has high fluidity and high bonding strength, and good impermeability and volume stability, and can significantly improve groutability and reinforce performance of subtle fracture of rocks if applied.