Abstract: Due to fact that the conventional clay oil based drilling fluid used in shale gas horizontal wells in the Changning Block, Sichuan Basin, is poor in rheology and easy to induce lost circulation, the clay-free oil based drilling fluid has been studied. To enhance the electric stability and suspension performance of the proposed oil based drilling fluid, the composite emulsifier G326 and oil-soluble polymer viscosifier G336 were developed, and determined the formula for the clay-free oil based drilling fluid. Indoor test results showed that the clay-free oil based drilling fluid had a higher electric stability and lower final gel strength than conventional clay oil based drilling fluids, which is faverable for eliminating the thickening problem of oil based drilling fluids and lost circulation in complicated formations in the case of high density. When the clay-free oil based drilling fluid was deployed in four shale gas horizontal wells in the Changning Block, drilling operations were performed smoothly with stable borehole wall, no tight hole and sloughing, smooth tripping and clean wellbore, average ROP enhanced by 37.8%. Research results showed that the clay-free oil based drilling fluid could prevent the lost circulation that caused by conventional oil based drilling fluid with high density due to its high structural strength, which can meet the need of drilling shale gas horizontal wells fast and safely in Changning Block.