Abstract: Transformer windings are prone to deformation under impacts such as short-circuit currents and overvoltages, posing a serious threat to the security of power grids. Traditional off-line detection methods require equipment outage and cannot reflect the real-time condition of the windings, making the development of on-line detection technology of significant engineering value. Therefore, this paper reviews the on-line detection technologies for transformer winding deformation and classifies the existing methods into four categories based on their detection principles: on-line leakage reactance method, vibration monitoring method, ultrasonic detection method, and on-line transfer function method. The paper particularly discusses the innovative concept of using the system"s own very fast transient overvoltage (VFTO) as an excitation signal in the on-line transfer function method, while also identifying the technical directions that still require further investigation for this approach.