Using floating-ground voltage sensors to measure the voltage of distribution network transmission lines greatly reduces cost and size compared with traditional voltage transformers. However, their capacitive voltage division principle makes measurement results susceptible to environmental interference, especially in rainy conditions. The power frequency measurement results of the sensor show rapid and significant fluctuations, which are easily confused with actual grid faults and pose challenges for distribution network operation monitoring. To address this issue, based on the sensor′s voltage division model and via theoretical analysis and simulation, this paper firstly based on the sensor′s voltage division model and via theoretical analysis and simulation, identifies that severe distortion of the sensor′s measured electric field in rainy conditions and interference to the ground-coupled capacitance of the sensor′s upper and lower plates are the main causes of the sensor′s measurement errors. Secondly, through analyzing the voltage division model and sensor transfer function, it designs a new-type voltage sensor with an equipotential shielding case and displacement current transimpedance amplification is designed. The equipotential shielding case reduces the impact of sudden external environmental changes on coupled capacitance, while the transimpedance amplification circuit and displacement current signal processing enhance anti-interference capability. Finally, an experimental platform is established, including an adjustable simulated rain device, a standard sine-wave voltage source, and a data acquisition system to test the new sensor and verify the rationality of its design scheme. Using standard 5.77 kV power frequency input voltage, comparative experiments under different rainfall intensities and simulated rain positions validate that the new sensor can effectively suppress rain interference, with measurement fluctuations reduced by over 50%. It also has good measurement accuracy, stability, and linearity, with the measurement error of the 5.77 kV power frequency voltage amplitude within 3.3%.