Abstract: During the drilling of Block Tuo 826 in the Shengli Oilfield, synthetic based drilling fluid should be used to protect the reservoir, but one more technical casing must be used for drilling fluid conversion. In order to avoid running another technical casing and reduce drilling costs, the conversion technology of synthetic based drilling fluid in open hole was researched. Based on the basic formula of synthetic based drilling fluid, a formula of plugging synthetic based drilling fluid was determined by optimizing the amount of oil based plugging material DF-1. And combined with field operation technologies, the conversion technology of synthetic based drilling fluid in open hole was developed. Evaluation was conducted on the plugging capacity and anti-contamination capacity of the plugging synthetic based drilling fluid and its effect on water based mud cake by means of visual sand-bed test, plugging capacity test, anti-contamination test and filter cake breaking test. It was shown that its pressure bearing capacity was over 7.5 MPa. When the invasion amount of the cuttings, well site water, water base drilling fluid and spacer?fluid reached 15%, its rheological property and electrical stability could also meet the design requirements. And this system could improve the quality of the existing cake by means of compression. The technology was successfully applied in 3 wells in Block Tuo 826 and the goals of wellbore stability and reservoir protection were realized. It was shown that the conversion of synthetic based drilling fluid in open hole was feasible. Based on this technology, drilling cost could be reduced significantly.