In complex drilling environments, the dielectric characteristics of the near-bit fluid (drilling muds) can directly reflect its content and nature of the solid-phase particles, liquids and gases. The dielectric constant and conductivity are two major electrical parameters of near-bit fluids, whose values make a difference to the amplitude and phase of measured signals, ultimately leading to insufficient accuracy in understanding fluid properties during drilling. This is one of the reasons why downhole gas kick and mud loss cannot be effectively controlled. Therefore, clarifying the dielectric properties of near-bit fluids is an essential job in identifying and interpreting complex operating conditions of downhole wells. Considering that the water content, temperature, and density of the fluid can affect its dielectric characteristics to varying degrees, this paper adopts a sensor with four-core annular electrodes to measure the dielectric characteristics of the near-bit fluid. Based on the finite element method (FEM), a simulation modeling experiment is conducted on the current density distribution of dielectric sensors, and a dielectric measurement system prototype is designed and built, mainly including dielectric sensors, a hardware control circuit, and an upper computer. The results show that the system can be used to access the dielectric characteristic parameters of the near-bit fluid, further verifying the feasibility of safety prewarning over the drilling process through dielectric characteristic monitoring.