Taiwan has a subtropics monsoon climate and frequently counters typhoons and rainfall, resulting in shallow landslides and landform changes. The significant quantities of soil yields affect the infrastructure safety and environment nearby. Among the influences of factors of safety of the soil slopes, the soil water content (WC) is crucial, and WC and electrical conductivity (EC) profilings of the soil slope assist early warning during a rainfall event. Thus, this study adapted time domain reflectometry (TDR) with the current bipolar type penetrometer to simultaneously measure the volumetric WC (VWC) and the EC in a case study at reservoir watersheds. TDR committed continuous monitoring but with apparent bias because of the insufficient sensing volume of the bipolar type penetrometer. Based on the field experience, this study applied numerical analysis to improve the TDR penetrometer into the dipole type by examining the electrical field, the electrical energy density, and the strain with external stress. After considering all the factors to obtain the optimal design, verifications were accomplished in a sandbox to retrieve measurement performance and the relationship between the soil VWC and the EC in different wetting and drying rates. Results denoted the adequate procedures for TDR on-site monitoring and corresponding calibration of soil VWC and EC for slope stability analysis.