Due to the complex processes and high cost for mineral surface modification of montmorillonite, and hard to meet the requirements for high-temperature resistance when used for drilling spacer fluids, this study adopted Na2CO3 +NaOH as a cheap composite modifier, and employed multiple modification methods including stirring, ultrasonic treatment, and microwave treatment etc. to sodium-modify natural calcium-based montmorillonite. The sodium-based montmorillonite was successfully synthesized, and the product showed good performance when applied at drilling spacer fluids. X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) were used to characterize the structure of the modified sodium-based montmorillonite, and the thixotropy, temperature resistance ability, and suspension stability of the sodium-modified montmorillonite were also evaluated. Moreover, the mechanism of sodium modification of montmorillonite by the composite modifier was proposed. The significant findings are: 1) when the mass ratio of the modifier to natural calcium-based montmorillonite reached 1:10, the calcium-based montmorillonite could be completely modified into sodium-based montmorillonite; 2) when the dosage of the modifier was reduced to half, completely sodium-modified montmorillonite was also obtained through microwave and ultrasound treatment. 3) the spacer fluid prepared with the sodium-modified montmorillonite had excellent thixotropy without supernatant precipitation after long time standing, specifically when the mass ratio of the modifier to montmorillonite was 1:20, the drilling spacer fluid prepared after microwave-assisted stirring modification exhibited strong high-temperature suspension stability, with a density difference of only 0.03 g/cm3 after water bath. This research is significant for reducing the cost for oil well cementing and promoting the high-value application of natural montmorillonite.