Under the influence of the environment, newly poured concrete will experience excessive evaporation of surface water, and the loss of concrete water content will lead to the deterioration of concrete performance. Therefore, the water change process inside the concrete at the construction site needs to be monitored accurately. However, existing technologies cannot fully meet this requirement. In this study, the theoretical relationship between water change and dielectric constant change inside the concrete was analyzed based on the dielectric constant characteristics of various components inside the concrete. Based on the principle of standing wave ratio, a real-time electrical measurement technology for in-situ water content was proposed. Results show a quadratic polynomial correlation between the measured voltage of the sensor and the water content of the concrete, and the experimental verification results show a correlation coefficient of above 0.95 between the measured values and the theoretical relationship. The application results in a water conservancy project site demonstrated that this method has good value for in-situ application and can accurately and automatically monitor the distribution of water content in the concrete in real time.