The invention relates to the technical field of spacer fluid for oil field well completion, in particular to oil-based mud spacer fluid for well cementation and completion and a preparation method thereof. The spacer fluid comprises a surfactant, a composite suspending agent, a weighting material and water. The spacer fluid, which is obtained by compounding according to the formula, has excellenthigh-temperature suspension stability and flushing efficiency, and has excellent compatibility with cement paste, oil-based mud and a mixture of the cement paste and the oil-based mud at a high temperature, so that mud residues on a well wall can be effectively removed after the oil-based mud spacer fluid is used for flushing before well cementation, the well wall is adjusted to be a hydrophilic surface, and the cementation quality of cement and the well wall in the later well cementation operation is improved; meanwhile, the phenomenon of high-temperature viscosity reduction or high-temperature thickening does not occur in the well cementation waiting process, a good cementation environment is continuously provided for cementation of well cementation operation cement and the well wall, and the cementation quality of the cement and the well wall in well cementation operation is further improved.