In this paper, the effects of the mass ratio of magnesium-rich nickel slag and fly ash, the content of composite alkali activator (sodium silicate-Na2CO3) and water binder ratio on the mechanical properties of magnesium-rich nickel slag-fly ash based geopolymer were studied by orthogonal test. The products were characterized by XRD, SEM, EDS and TG. The results show that the 28 d compressive strength of the optimal sample reaches 37.50 MPa. XRD results shows that calcium silicate gel form in the products of 7 d and 28 d. SEM and EDS analysis show that there are chabazite (N-A-S-H) and sodium magnesium aluminum silicate gel (N-M-A-S) amorphous gel phases in the products. These gel phases are the main reasons for the increase of the strength of geopolymer.