The invention discloses white ink pretreatment fluid as well as a preparation method thereof and a direct-injection digital printing process, which belongs to the technical field of textile digital printing. The pretreatment fluid is prepared from the following components: 1 to 5 parts of cation colloidal silicon dioxide, 3 to 7 parts of adhesive, 2 to 6 parts of drying inhibitor, 3 to 8 parts of multivalence alkaline-earth metal salt, 3 to 8 parts of color bleeding prevention agent, 1 to 5 parts of cation mordant, 0.1 to 0.5 part of sterilizing agent, 0.1 to 0.5 part of wet defoaming agent, and 60 to 80 parts of deionized water. All raw materials are successively fed into a material machine and are respectively and uniformly stirred and filtered to obtain the white ink pretreatment fluid. The pretreatment fluid has good dispersion stability, is uniformly smeared when in use, is high in adhesive force, can prevent the seepage of the white paint direct-injection ink on the surface layer of a dark fabric, can also effectively improve the concealing capacity and adhesive force of the white paint direct-injection ink on the dark fabric, and can greatly enlarge the application range of the white paint direct-injection ink.