In 2015, gas hydrate trial mining horizontally butted well was constructed in Qilian Mountain Muli permafrost area, The formula of flushing fluid was designed optimally according to the formation conditions such as hydration, collapsing and chipping. Good wall protection and lubrication results were achieved by adjusting the dosage of drilling mud additives reasonably. Mud parameters such as density, viscosity, sand content, filter loss and some others were monitored in the application, and the properties of flushing fluid were timely adjusted. The application results show that the flushing fluid has good comprehensive properties and can meet the requirements of trial mining well drilling construction.