To meet the requirements of low-density cement slurry for low leakage easily wells, developed a set of fly ash composite low- density cement slurry system which the density range in 1.40 g/cm ?- 1.55 g / ?cm. When fluid loss additive 11%, retarder dosing amount for 0.05% and activator for 1%,the compressive strength of the fly ash composite low- density cement slurry system is over 18MPa ,and the water filter can be controlled with in 50 mL, initial low viscosity, thickening time adjustable, comprehensive performance is good.