Abstract: A filtration property of the drilling fluid is very important.By the study of filtrate reducer used in water-in-oil emulsion drilling fluid,it is shown that there is the same mechanism in the oil-based drilling fluid as in the water-based drilling fluid,and excellent linear relationship between the filter loss and the square root of the time。Evaluation method of filtrate reducer in water-in-oil emulsion drilling fluid is same as water-based drilling fluid,and electric stability for the water-in-oil emulsion drilling fluid should be measured.After hot rolling under 250℃/16h high temperature,the fluid loss of API for the water-in-oil emulsion drilling fluid decreases greatly.