Study on the mechanical properties of alkali-activated materials is able to promote its practical application in engineering. In this paper, the effects of slag content and fine aggregate content on the compressive strength, elastic modulus and stress-strain curves of alkali-activated paste/mortar were investigated. The results show that the addition of slag significantly improves the compressive strength and elastic modulus of alkali-activated paste/mortar. The addition of fine aggregate reduces the compressive strength while improves the elastic modulus. The elastic modulus of alkali-activated slag/fly ash paste ranges from 12.83 GPa to 19.53 GPa at 28 d. The elastic modulus of alkali-activated slag/fly ash mortar ranges from 18.72 GPa to 23.10 GPa at 28 d. When the amount of fine aggregate is 40% (mass fraction), the peak stress and peak strain decrease obviously while elastic modulus increases. The piecewise equation is used to fit the stress-strain curves of alkali-activated slag/fly ash paste/mortar. The fitting curves are in good agreement with the measured curves. The fitting results indicate that the stress-strain curves of alkali-activated slag/fly ash paste/mortar become steeper with the increase of slag content and the curing age, reflecting the brittleness of materials, which is consistent with the rising curves.