Abstract: Spacer is one of the crucial factors influencing the quality of wellcementing. It is particularly important to choose right ingredients of the spacerfor well cementing in sea-water drilling fluid. The paper proposes a new recipe of spacer and studies itsperformance in laboratory. The experimental results demonstrate that the newspacer works well with the environmental fluids, such as sea-water drillingmud, cement slurry and salty water. It can stabilize the wall ofthe well. And, with carefully chosen solid particles and surfactantsin the recipe, the spacer fluid can efficiently used to wash the wall of thewell, especially, for the mud cake. Furthermore, in-situ experimentswereconducted in Shengli Oilfield, Shangdong Province, China. The results showthat the new spacer fluids can shorten the mixing length of the drilling mudand the cement slurry, and can evidently improve the quality of the secondinterface of the cement annulus.