Abstract: In order to study cuttings erosion efficiency rule for well washing and drill pipe rotation in slim-hole horizontal wells, a similar experiment design was carried out to study cuttings start-up velocity and cuttings bed erosion efficiency according to the washing well condition of long horizontal wells in Mahu Triassic deep formation. It could be found that the cuttings bed in different flow rates show three distinct transport modes: stable cuttings bed movement, sand dune wave transport and cuttings bed overall transport. For the mode of stable cuttings bed movement, the effects of cuttings start-up velocity and drill pipe rotation on cuttings bed height and required stability time under different cuttings size, flow rate, and pipe rotation speed are analyzed. For the mode of the overall cuttings bed transport, the effects of cuttings size, flow rate, and initial length on the overall cuttings bed transport velocity are analyzed. Results indicated that the smaller the size of cuttings is, the smaller the required start-up velocity is. When the drill pipe rotates, the cuttings start-up velocity decreases by about 50%. The increased pipe rotation speed can reduce the required time for cuttings bed to become steady and improve the efficiency of the cuttings transport. For the mode of the overall cuttings bed transport, the effect of cuttings size on the erosion velocity is negligible at a lower flow rate, and the transport velocity of large cuttings size is fast at a high flow rate. Flow rate and pipe rotation are the keys to improving the efficiency of cuttings erosion. The drill pipe rotation speed in the existing well cleaning construction is low, which can be appropriately increased to improve the efficiency of well cleaning.