© 2015 Society of Chemical Industry. To construct an ultra-high temperature stable drilling mud with a relatively low viscosity, a new viscosity-reducing additive was synthesized and evaluated. This additive is a quadripolymer of acrylic acid (AA), 2-acryl-amido-2-methyl-1-propane sulfonic acid (AMPS), methyl acrylate (MA) and dimethyl diallyl ammonium chloride (DDAC) with low molecular weight synthesized by free radical polymerization, and its general molecular composition was confirmed by Fourier transform infrared spectroscopy. Thermal gravimetric analysis showed that the polymer has a very high temperature tolerance, i.e. it is stable up to 340 °C. Turbidity measurements with a UV-visible spectrophotometer showed that the polymer has a high salt tolerance in solution. It remained in solution at NaCl concentrations of up to 266 958 mg L-1 and at CaCl2 concentrations of up to 5000 mg L-1. The quadripolymer was found to inhibit hydration of the exchangeable ions in the clay particles and this was affirmed by a reduction in interlayer spacing. It also displayed high adsorption on calcium montmorillonite at different pH and even in the presence of NaCl, and caused the breakup of the particle network structure. This additive, when added to drilling mud prepared in both fresh and salt water, was found to reduce the mud viscosity and yield stress before and after aging for 16 h at 245 °C compared to base mud especially below 220 °C and 4.5 MPa. Importantly, this additive in mud was found to perform effectively at a relatively high pH and in a broad pH range of 7-11.