AD205H is the first simplified three-spuding horizontal well in Al-ahdab oilfield of Iraq. Upper water sensitive mudstone is easily being dispersed and mud forming with diameter reducing; high pressure salt water layer, chert nodule beds and oil reservoir with high content of limestone increase the construction cycle and restrict the application of horizontal wells in this area. Aiming at the above difficulties, evaluation tests on the drilling fluid inhibitory, temperature resistance, salt resistance, anti pollution were made, the mixed oil polysulfonate drilling fluid system was developed. Field application shows that polysulfonate drilling fluid system has strong antipollution ability with stable performance in the construction process, which has strong inhibition on water sensitivity mud of the upper part and very good sealing effect for lower limestone formation; downhole accidents are reduced.