Abstract: Two kinds of bentonite with the ethylene blue adsorbed of 35.2 g/100 g and 23.6 g/100 g from Luoyang, Henan Province were Na−modified and mixed with synergists to develop civil engineering mud. The effects of sodium agent and synergists on the mud properties of bentonite were systematically studied. The results showed that the effect of Na−modification on the viscosity of high−grade bentonite with the ethylene blue adsorbed of 35.2 g/100 g was more significant, and the viscosity of high−grade bentonite was increased from 3 mPa·s to 34 mPa·s by 4% Na₂CO₃ semi−dry modification. The high−grade bentonite could reach the specification of high−viscosity bentonite after the Na−modification by addition of 0.75% Na−CMC. It was found that the combination of SPA and different synergists could effectively improve the viscosity and reduce the filtration loss and yield point/plastic viscosity ratio. The low−grade bentonite with the ethylene blue adsorbed of 23.6 g/100 g could reach the specification of medium−viscosity bentonite by addition of 0.2% SPA+1% PAC.