The invention discloses a method for testing the content of oil sludge in phosphate fire-resistant oil. The method includes the following steps that 20 g of the to-be-tested phosphate fire-resistant oil is added into a constant-weight centrifugal tube, n-pentane is added into the centrifugal tube, an opening of the centrifugal tube is plugged through a first plug, and then the material is mixed to be uniform; 50 mL of n-pentane is added into the centrifugal tube, after a second plug is plugged into the opening of the centrifugal tube, shaking is conducted, the centrifugal tube is put in a centrifuge for centrifugation, and after centrifugation, an upper solution of liquid in the centrifugal tube is poured into filter paper for filtering; the liquid in the centrifugal tube is put in a flask; the filter paper is washed with petroleum ether till no oil sample trace exists on the filter paper, and then the filter paper is put in a weighing bottle; the flask and the weighing bottle are dried and then cooled, the cooled flask and the cooled weighing bottle are weighed, and the content w of the oil sludge in the to-be-tested phosphate fire-resistant oil is obtained. The content of the oil sludge in the phosphate fire-resistant oil can be accurately tested.