Abstract: The plant′s thickener underflow is considerably high in density caused by the excessively high working resistance of thickener′s rake. To tackle this problem, following an analysis of the slime water density as well as the size, density and mineral composition of solid particles in the water, test is made of the natural sedimentation characteristics of the slime water. Test result shows that the considerably high content of magnetics of thickener underflow is the main factor leading to the difficulty of the settlement. Finally, the measures and suggestions for avoiding the jamming up of the thickener rake are proposed.