In order to evaluate the chemical stability of magnetite tailings in alkaline environment of cement, the influences of magnetite tailings on pH value of saturated Ca(OH)2 solution, expansion rate of mortar bars, and corrosion rate of reinforcement were studied. The results show that the magnetite tailings with particle sizes of 2.500 and 1.250 mm have more significant effect on the decrease of pH value of saturated Ca(OH)2 solution in early stage. With the extension of age, the effects of magnetite tailings with particle size below 1.250 mm on the decrease of pH value become more significant. When magnetite tailings with different particle sizes replace parts of natural sand, the expansion of mortar bars is different. The influences of magnetite tailings with particle sizes of 0.630 and 0.315 mm on the volume deformation of mortar bars are more significant. Moreover, the mortar bar expansion rate of magnetite tailings with particle size of 0.630 mm is the largest after a longer age. In a mixed solution of Ca(OH)2 and NaCl, the magnetite tailings react more readily than reinforcement, which can slow down the corrosion of reinforcement.