Abstract: Part key blocks of Dagang Oilfield belong to medium porosity, low permeability reservoir that is easily damaged.According to the systematic research on the basic characteristics of the reservoir and damage mechanism for these key blocks, thispaper optimized the drilling and completion fluid system of the ideal packing type and the broad-spectrum oil-film temporary blockingtype. Further more, the reservoir protection effect with two types of system was contrasted by the laboratory test and field application.The results show that the broad-spectrum oil-film temporary blocking type is preferable to the ideal packing type on the effect ofprotecting formation. Plugging rate of permeability was over 91.4%, permeability recovery value was up to 99.3%. The pure damagedskin factor of field test was close to zero. It proved that the formation damage resulted from the solid particles and filtrate in the drillingand completion fluid was prevented, achieving better protection to the formation.