Spherical polystyrene hollow microsphere was prepared by emulsion polymerization method. The surface of microsphere was compact and smooth and shell thickness was uniform. The density of the particle was 0.2—0.6 g/cm3 and the diameter was 10—120 μm. In laboratory research, microsphere which density of the particle in average was about 0.50 g/cm3 and the diameter was in range of 20—60 μm was chosen as light weight additive. Low-density (0.92 g/cm3) drilling fluid was prepared by adding 10% hollow polystyrene sphere to 0.2% xanthan gum solution. The density of drilling fluid changed to 0.96 or 0.93 g/cm3 when it was rolled 16 hrs at 90℃ and under constant pressure or 0.3 MPa. Drilling fluid’s density changed little when temperature rose to 90℃ and pressure rose to 50 MPa. Low-density drilling fluid’s density was 0.92 g/cm3 when it was stirred 8 hrs at a speed of 12000 r/min, and that was 0.93 g/cm3 when it was grinded 8 hrs, which indicated that the influence of stirring and grinding on hollow polystyrene sphere was smaller than that on hollow glass sphere. Hollow polystyrene sphere was applied in oilfield for the first time at Zhangwu area in northeast. The results showed that polystyrene sphere had little effect to drilling fluid and had a good ability to reduce drilling fluid’s density. But the sphere was easy to reunite together and adsorb to drillings, as a result, it was easy to be screened by solids control equipment.