Abstract: In consideration of the fact that, during drilling of horizontal wells with oil-base mud in the pilot area for development of shale gas in our country, down-hole troubles like severe wellbore instability and lost circulation frequently occur, the outcrops of Longmaxi shale in the southeast of Chongqing were taken as experimental objects in order to carry out the tests of shale rock mechanical properties before and after immersion in drilling fluid; based on finite element numerical model, a model was built for predicting the dynamic fracture under the condition of single vertical fracture connecting the wellbore, the behaviour of shale fracture width variation after immersion was studied, the method for prediction of dynamic fracture width based on finite element was corrected and improved, and the application of fracture width variation simulation was discussed to the protection of leaking reservoirs and control of wellbore instability. The results show that, after immersion in drilling fluid, the elasticity modulus of shale decreased, the Poisson’s ratio increased and the rock strength reduced significantly; after immersion, the dynamic shale fracture width was more sensite to changes in wellbore pressures; the half-fracture width increment under posite pressure differential of each fracture length and wellbore all increased by about 8% compared with pre-immersion; and the increment of half width increased greatly with the increase of posite pressure differential in the wellbore, and the dynamic fracture width could be up to several millimeter order; the three factors: drilling density, fracture width and bridging grain sizes were closely related. The parameters of dynamic fracture width obtained by the research work are very significant for selecting the LCM materials and grain size grading, controlling proper drilling fluid density and hence more effectely protecting the reservoir, controlling lost circulation and wellbore instability.