Abstract: In response to the development requirements of small-sized gaseous tritium light sources, a wet coating and sintering process for preparing micron-sized ZnS(Cu) phosphors coatings on the inner wall of a slender glass tube was independently developed. And a coating device with controllable pressure, flow and temperature was designed and established. The coating device was used to explore the preparation process of fluorescent coatings for slender glass tubes with a specification of φ1.1 mm×550 mm, and to carry out research on the influence of slurry formulations on the viscosity of mixed liquid formulations and coating thickness. By optimizing the drying parameters, the coating was finally obtained in the slender glass tube. The cross-sectional SEM analysis shows that the coating has good compactness, and its thickness is (16.9±0.6) μm(n=32). The coating has good uniformity in thickness and is stable and does not fall off after being placed for a long time. The luminescence brightness of the tritium light tube sample obtained by this process is 0.642 cd/m², with excellent uniformity. This provides a basis for device conditions and process optimization for the development of subsequent tritium light source products.