During the drilling of near-horizontal wells in phosphogypsum tailings ponds, borehole instability is prone to occur due to the loose structure, low compressive strength, strong permeability, and easy hydration of phosphogypsum. To ensure borehole wall stability, the anti-calcium and anti-collapse drilling fluid system LDP-A was developed by analyzing the borehole wall instability mechanism and optimizing drilling fluid additives. For severely fractured phosphogypsum tail sand layers, the cement slurry SP-A for wall protection was designed. The results indicate: (1) The phosphogypsum surface is well-developed with fractures, and its strength decreases significantly after water exposure. It is prone to induce collapse and falling blocks, and leads to instability of the near-horizontal borehole wall. (2) The optimized drilling fluid LDP-A exhibits excellent rheology and inhibition, with a plastic viscosity of 22mPa·s, a yield point of 7Pa, and a fluid loss of 6.2mL. After adding 1%~5% phosphogypsum (simulated drill cuttings), the rolling recovery rate increases from 92.34% to 93.44%, and the buckling hardness increases from 52N·m to 56N·m. (3) The wall-protection cement slurry SP-A demonstrates good pumpability, high compressive strength of the consolidated body, and strong contamination resistance, with a fluidity of 228mm and compressive strengths at 1d, 3d, 7d, and 28d of 7.21MPa, 19.73MPa, 25.42MPa, and 38.71MPa, respectively. After adding 1%~5% phosphogypsum, the compressive strengths at various ages decrease by only 13.59%, 9.73%, 11.37%, and 8.13%, respectively, and the fluidity decreases by only 2.63%. Field tests show that the LDP-A drilling fluid has significant wall-protection effects and outstanding calcium contamination resistance. This research provides a valuable reference for drilling fluid studies in similar formations.