Viscosity is one of the important parameters to evaluate the mud property of bored pile. Because of the large pile diameter, low mud circulation velocity and low shear rate, the methods of evaluating mud rheological property of petroleum drilling and geological exploration is not suitable for evaluating bored pile. This paper firstly analyzes the circulation velocity of bored pile to calculate the shear rate distribution; then analyzes the shear rate of rotary viscometer and suggests that variable speed viscometer with low rpm is more suitable to evaluate the mud of bored pile; the plastic viscosity, effective viscosity and yield point formulation under low shear rate were finally deduced.