Abstract: At present, the return of drilling mud is measured by target type meter, but the result is not very accurate because of environment. The ultrasonic flowmeter is the most accurate, normal and sensitive apparatus, but the return flow of drilling mud can not be measured by ultrasonic flowmeter because of the huge ultrasonic attenuation in drilling fluid. The ultrasonic attenuation law was researched with the Urick Model when the flow of drilling fluid was measured by ultrasonic flowmeter. The effects of the factors on ultrasonic attenuation were analyzed, such as ultrasonic propagation distance, frequency, and liquid density..It was concluded that the ultrasonic attenuation would increase by one order of magnitude with the increase of the quantity of solid-phase pellet in the drilling fluid by one order of magnitude, and the ultrasonic attenuation increased with the increase of the diameter of solid-phase particles in the drilling fluid, the drilling fluid density and the propogation distance. The larger ultrasonic frequency is, the quicker ultrasonic attenuation is.