This paper presents a non-contact over-voltage sensor of transmission lines by utilizing the stray capacitor between transmission lines and the over-voltage sensor capacitor to form a capacitor over-voltage divider. The stray capacitance is calculated by the series-parallel method. The factors that affect the sensor accuracy， the structure design and electromagnetic interference shielding of the over-voltage sensor are analyzed. The results of power frequency voltage and lightning over-voltage tests indicate that the sensor has good precision and linearity that meets the requirements for over-voltage on-line monitoring.