Abstract: The muddy experiment on 50.5 mm different density raw coal from Dingji Coal Preparation Plant of Huainan Mining Area was conducted.The X-ray diffr actor and the electrophoresis experiment was individually applied to measure the raw coal mineral composition and the zeta potential on the micro particle surface of th e slime.The results showed that the main mineral composition of the raw coal to be prepared would be including quartz, kaolinite, green mudstone and others.The mudd y rate of the high density and medium density raw coal was high.In the muddy slime of the +1.60 g/cm3 raw coal, the-0.045 mm particles would be the majority and the- 0.025 mm micro particles would be over 90%.With the raw coal density increased and the slime particle reduced, the zeta potential on the micro particle surface of the muddy slime would be reduced in potential.The water hardness, the soluble salt in the raw coal and the particle surface property would have important influence to the zeta potential occurred on the slime micro particle.The zeta potential increased on the slime micro particle surface would cause the particles in the slime water in more stable dispersed state.