The slag-clay mixtures made of construction waste slag mixed with certain proportion clay have been used as filling materials, which have significantly social economy and environmental protection benefits. In order to reveal the compaction properties of slag-clay mixtures, a series of compaction tests are conducted to investigate the influence of the slag content and initial gradation on the behavior of compaction and particle breakage. The results show that the optimum moisture content is smaller for slag-clay mixtures than for clay, and the maximum dry density is larger for slag-clay mixture than for clay. As the slag content increases, for the slag-clay mixtures the optimum moisture content increases, and the maximum dry density increases firstly and then decreases. Therefore there exists the optimum slag content of 30% in the slag-clay mixtures despite the different initial gradation of slag. The relative breakage of slag particles increases with slag content during the compaction. The relative breakage is greater for uniformly graded than for well-graded slag, but is not much affected by the initial particle size of slag at the same slag content. The maximum dry density is greater for larger size than for smaller size particles of slag, which leads to well compaction of wide-graded slag-clay mixtures. The compaction properties of slag-clay mixtures are explained by linking the structural response of slag-clay mixtures with the slag content and behavior of particle breakage.