Considering the characteristics of reservoir and open hole completion with horizontal well development in Iraq Missan oil fields，a non-gel breaking liquefiable drill-in fluid was designed. By examining the effect of addition of flow pattern regulator VIS-B，fluid loss reducing agent STARFLO and lubricant Lube on drill-in fluid performance，the basic formula of the non-gel breaking liquefiable drill-in fluid system was determined，and the anti-temperature performance，anti-formation water pollution ability，inhibition performance and completion fluid's ability to break mud cake of drill-in fluid system and its protection effect on reservoir were evaluated. The research results showed that the formula of drill-in fluid system was fresh water+0.2%NaOH+0.2% Na2CO3+16%NaCl + 3%KCl+1.0%VIS-B+ 3% STARFLO+0.5% DUALSEAL（alkalinity regulator）+5% JQWY（acid-soluble temporary blocking agent）+ 1.5% Lube+40%HCOONa（1.28 g/cm3），and the temperature resistance of the system could reach 130°C，the rolling recovery rate reached 95.34%，and it had a good resistance to high-mineralized formation water pollution. The filter cake formed by the system could be dissolved under acidic conditions and did not require gel breaking treatment. Reservoir protection effect evaluation showed that the permeability recovery rate of the formation cores fouled by this system could reach 97.58% after the matching completion fluid treatment. The drill-in fluid system did not require gel breaking treatment which could reduce operating procedures and operating costs，and improve production efficiency.