This paper presents the design and geometry optimization of a ring conductance probe for measuring the conductance of oil-water mixtures in horizontal pipes. Using the finite element method (FEM), we first investigate the sensitivity distribution of the electric field generated by a pair of ring-shaped exciting electrodes, and calculate the static response on the measurement electrodes under horizontal oil-water flow patterns; we then figure out the optimum geometry dimension with minimum deviation from linearity and high spatial resolution. Finally, we carry out the flow loop test in a horizontal oil-water two-phase flow pipe to obtain the measurement response of the ring conductance probe, and conclude some advantages for measuring liquid holdup with the newly designed conductance method.