Abstract: In order to research the variation law between hardness and conductivity of coal slurry at Xingtai coal preparation plant,studied the coal slurry water system in this plant.The samples collected from circulating water,flotation feeding and the first stage overflow of thickener,and use the conductivity method continuously measured the water hardness and electric conductivity of the samples.The test results show that the after adding the agent,water hardness and electric conductivity of coal slurry system both increased at the beginning,and then decreased,after that hold steady,the variation law of the curves basically the same,and have synchronism.After stable of the coal slurry system,the variation of water hardness and electric conductivity basically the same,and can be researched as a completed and independent system.