Abstract: There are a lot of low-grade calcium bentonite in Shangtianti area of Xinyang, Henan Province. The Φ600 of mud was 2 mPa·s and the filter loss was 89 mL, which can not meet the technical requirements of drilling grade bentonite(GB/T 5005-2010, Φ600 ≥ 30 mPa·s, filter loss ≤ 16 mL). Sodium modified bentonite was prepared by semi-dry modification process, when the dosage of sodium carbonate was 2.5% and sodium time for 1.5 hours, the mud filter loss decreased to 31 mL. Then, the influence of synergist on the properties of bentonite slurry was studied by single factor experiment. XRD, FTIR and SEM potential were used to investigate the mechanism of the effect of the synergist on the slurry-making properties of Bentonite. Finally, the synergist was optimized and regulated through orthogonal experiment. The results showed that the synergist facilitated the formation of network structures. when the composite synergist composed of 1% MgO and 1% APAM, the viscosity reached 34 mPa·s and the filter loss could be reduced to 12 mL, which could meet the technical requirements of bentonite for drilling.