The invention discloses a device and method for mixing and adding a chemical reagent to paste filling slurry. The device comprises a slurry conveying pipeline, a chemical reagent conveying pipeline, achemical reagent mixing pipeline, a chemical reagent ejecting pipeline, a chemical reagent ejecting hole and a cylindrical chemical reagent ejecting pipe, the chemical reagent and the paste slurry are separately conveyed for adding the chemical reagent conveniently, the mixing effect is good, the problems of the reasonable and efficient performance of the way of adding a filling paste additive and an accelerator and the insufficient mixing of the slurry and the chemical reagent are solved, thus the mixed paste can be quickly solidified when filling, the residue of the paste in the pipeline isreduced, and the occurrence of pipeline blockage is prevented. An existing slurry adding pipeline and an existing chemical reagent adding pipeline are combined, the chemical reagent and the accelerator are added to the slurry, and meanwhile the slurry is fully mixed with the paste. The method is easy to operate, the chemical reagent is conveniently added to the slurry, the mixing efficiency of the paste materials is improved, the cost is low, and the mixing effect is good.