In order to obtain the optimal mixture ratio of geopolymer concrete with the best mechanical properties, the optimization method of mix ratio of geopolymer concrete with fly ash and slag as precursors was systematically. The water/binder ratio, alkali activator modulus (the ratio of SiO2 and Na2O molecules in alkali activator) and slag content (the mass fraction of slag in precursor) were selected as variables to design the response surface scheme for compressive test of geopolymer concrete. The response surface model of compressive strength was established by statistical method, and the fit between model and test results was verified. The results show that the water/binder ratio has a significant effect on the compressive strength of geopolymer concrete, and the best choice of water/binder ratio is 0.376. The effect of slag content on the compressive strength of geopolymer concrete is extremely significant. The higher the proportion of slag instead of fly ash is, the higher the compressive strength of geopolymer concrete is. The optimum content of slag is 48.2%. The modulus of activator has no significant effect on the compressive strength of geopolymer concrete. The water/binder ratio affects the pore structure of matrix, and the slag affects the macroscopic mechanical properties of matrix by changing the compactness of matrix structure.