Abstract: Stability of gypsum whisker at the different solutions was studied. By measuring compression strength, rupture strength, impact work, hydration heat, and through the tests of mercury injection, X-ray diffraction, and scanning electron microscopy, the effect of gypsum whisker on the mechanical properties, pore structure, and hydration heat of hardened fly-ash cement paste have been analyzed. Results show that gypsum whisker could improve strength of fly ash cement composite material, especially early strength, and the big pores got smaller, but the total porosity and density increased. The hydration heat of fly-ash cement could be reduced by adding appropriate gypsum whisker.