Abstract: Being affected by formation temperature and fluid column pressure, it is difficult to predict the equivalent static density (ESD) of water-based drilling fluids. It is more difficult to predict the downhole ESD variation law of water-based fuzzy ball drilling fluid due to the ball cores wrapping a certain amount of gas, making the formation temperature and fluid column pressure affect not only the density of based liquid, but also the volume of ball cores. A downhole ESD prediction model for fuzzy ball drilling fluids in different well depths is established by firstly performing a laboratory PVT experiment to test the density of the solid-free fuzzy ball drilling fluid with standerd state density of 0.85 g/cm3, under the pressure range of 1~20 MPa and the temperature range of 30~130 ℃, and then processing the 220 density data points with the multiple regression method. The correlation coefficient is 0.96 and the relative error is below 5%, and the reliability is high. There is a 4% difference between the density calculated by the model at the depth of 2500 m and the actual measurement result in M80-C1 test well, indicating that this model can be used in downhole ESD prediction.