The main purpose of this article is to replace NaOH with NaCl as slag hydration activator, and preliminarily study the hydration mechanism. By strength test, EDS and XRD analysis, we studied the hydration products of slag composite cementitious material mixed with NaCl. The strength test results showed that the N-SM agent’s strength can reach 32.5 P•S standard. The EDS test results proved that the Cl- in NaCl reacted with the slag composite cementitious material and generated crystal of 3CaO•Al2O3•(0.5CaCl2•0.5CaSO4)•12H2O. At the same time, the Na+ generated NaOH and provided better alkaline environment to hydration of slag. The XRD test results proved that the slag generated zeolite mineral 0.8CaO•0.2Na2O•Al2O3 and C-S-H gel (Ca/Si=2) under alkaline environment.