Abstract: With the widespread application of capacitive voltage transformers (CVTs) in high-voltage power systems, accurate assessment of their operational status is crucial for ensuring grid safety, stability, and economic operation. Traditional periodic power-off offline inspection methods suffer from low efficiency, poor real-time performance, and failure to reflect the actual operating conditions of equipment. To address this issue, this paper designs an online monitoring device for CVTs. The device integrates a voltage signal conversion unit, voltage data acquisition unit, edge computing processing unit, and human-machine interface interaction unit. By paralleling the CVT secondary circuit, the device collects voltage waveform data in real time, constructs a virtual standard meter using edge computing technology, and dynamically evaluates the ratio error and phase angle error of the transformer, enabling online monitoring and early warning of metering performance. The device features multi-channel synchronous acquisition, high-precision analog-to-digital conversion, data modeling and analysis, real-time status display, and remote communication capabilities, allowing seamless integration with existing electricity information collection systems. Practical applications demonstrate that this device can achieve precise assessment of CVT operational errors without power interruption, providing an effective technical solution to replace traditional periodic inspections and enable condition-based maintenance, significantly improving operational efficiency and grid reliability.