Abstract: In order to investigate the cuttings removal rule of rotary drill pipes in slim-hole horizontal well washing, a similar test design was carried out according to the washing condition of long slim-hole horizontal wells in the deep Triassic system of Mahu Sag, so as to investigate the cuttings start-up velocity and cuttings bed removal efficiency. The test found that the cuttings bed presented three transport modes: stable transport, dune wave-type transport, and overall transport when washed with different flow rates. The effects of particle size of cuttings, flow rate, and rotational speed on start-up velocity of cuttings in stable transport state of cuttings bed were studied through tests, as well as the effect law of the rotational speed of drill pipe on the height of cuttings bed and the time required to form a stable cuttings bed. The effects of particle size of cuttings, flow rate, and initial mass of cuttings bed on the removal efficiency of cuttings bed in overall transport state were investigated. The results show that the smaller particle size of cuttings indicates a smaller start-up velocity, and the equivalent start-up velocity of cuttings will be reduced by 45.5% after the drill pipe rotates. The increase in rotational speed of the drill pipe will shorten the time required to form a stable cuttings bed and improve the removal efficiency of cuttings. When the cuttings bed is in overall transport state, the removal speed of the cuttings bed is less affected by the particle size of the cuttings at low flow rates, and the transport speed of the cuttings bed with large particle size is faster at high flow rates. The flow rate and rotational speed of the drill pipe are the key to improving the removal efficiency of cuttings during well washing process. However, the rotational speed of the drill pipe is currently low during well washing process and can be appropriately increased to improve the well washing efficiency.