Abstract: In order to regulate the water hardness of coal slime water to the benefit of both settling and flotation, realization of water hardness online monitoring is a basic guarantee. Since the technique of water hardness monitoring has not been industrialized, a soft-sensing technique which uses electrical conductivity as the auxiliary variable was proposed. Laboratory simulation and industrial scene monitoring tests were performed, and the results show that there exists linear correlation between the electrical conductivity and the water hardness, and the squares of linear correlation coefficient are 0.9796 and 0.9537, respectively. A soft-sensing model between the water hardness and the electrical conductivity were developed, through which the data of electrical conductivity can be processed and converted so as to realize online monitoring of the hardness of coal slime water.