Regarding to the technical problems of loose phosphogypsum hole wall collapse， dispersion of phosphogypsum encountering water， calcium contamination of drilling fluid and difficulty in drilling trajectory control during horizontal hole drilling in phosphogypsum formation， the research on drilling fluid technology was carried out indoors with analysis of the requirements of horizontal hole drilling in phosphogypsum formation for the performance of drilling fluid. The calcium-based film-forming environment-friendly drilling fluid was developed and successfully applied to horizontal hole drilling at a depository of phosphogypsum tailings in Guizhou. Laboratory test and field application results indicate that the system has good resistance to calcium intrusion， low filtration loss， and relatively stable rheological properties. A relatively strong protective film can be formed on the core surface， which can effectively prevent the hole wall collapse and protect core. In addition， the system consisting of non-toxic and biodegradable materials has good environment protection performance.