The present invention relates to a formula of gelatinizing material used in mortar and its preparation process. It uses natural volcanic tuff or pozzolan sedimentary rock to make that pass through drying dewatering treatment, and then mixes with calcareous oxide or compound and a moderate amount of gypsum as the excitation materials in the above-mentioned material. After having been mixed, those are pulverized to 4900 meshes/CM2, and its sieved residue is below 13%, so that the invented gelatinizing material is made up. The stickiness of dry mortar made by use of said gelatinizing material is large, and its adhesive power is strong.