Fly-ash is an industrial waste material, and is often used as a filling material in subgrade engineering. Many factors affect engineering properties of fly-ash. To investigate the effect of water content and age on the strength of fly-ash, a series of direct shear tests on a fly-ash are carried out. Relations between water content or age and shear strength with the same dry density were obtained. The results show that the shear strength and cohesion of the fly-ash change with variation of the saturation degree and increase with age, while the internal friction angle changes a little. Finally, shear strength of unsaturated fly-ash is predicted using unsaturated soil mechanics and soil-water characteristic curve (SWCC).