Abstract: Abstract: Heavy medium vessel is widely used in thermal coal separation. The flow field in the shallow tank significantly affects the separation effect. In this paper, Fluent software is used to study the flow field inside the W26F54 heavy medium vessel, and the critical sorting granularity at different positions of the shallow tank is calculated. The results show that there are eddy currents and reflux in the suspension in shallow tank. The flow ratio of horizontal flow and upwelling flow significantly affects the velocity field and density field in the shallow tank. Too large or too small the flow ratio is not conducive to material separation or transportation. When the flow ratio is 8:2, the distribution of velocity field and density field in the shallow tank is more ideal, which is beneficial to material separation and transportation of suspended products. The critical separation particle size in different regions of the shallow tank is different, which decreases first and then increases with the increase of shallow tank height.