Abstract: In order to further reduce the high density fluid loss and raise the application effect, the finish level for the high density mud should be strict control. Based on a lot of experiments, this file analyzes the influences regular pattern and incidence caused by the contents of bentonite, adding product, fluid loss agent, NaCl and caustic soda. The result shows that the contents of caustic soda is the main factor of influencing the mud filtration, secondly the fluid loss agent and NaCl, but the contents of bentonite and the kind of weighting materials have no obvious effect on the mud filtration. Controlling the pH between 8.5 and 9.5 after heating with high temperature we will got the minimum fluid loss. The results will provide reliable foundation for successful field application.