Abstract: The traditional way to clean the crude oil settling tank is to stop and empty the tank when the deposits accumulated a certain height and then use manual or mechanical methods to clean up, which not only affects the function of the tank, but there is a huge safety risk. Based on analyzing the settling of solid particles and argillaceous cement and the formation law of mud cake in a crude oil settling tank, a new on-line automatic sludge discharging device for large tank is designed. Designed according to the process principle of on-line timing rotary automatic sludge discharging and tank cleaning and external drive structure, the new device has a simple structure and is convenient to control and reliable to operate. More than 6 years of practical application results show that the device can meet the requirements of automatic sludge discharging in crude oil settling tank.