Abstract: A reflective Sagnac interferometer-type fiber optical voltage sensor based on the converse piezoelectric effect of quartz crystal was demonstrated. The sensor head consisted of two polarization- maintaining fiber sections with equal length bonded to the circumferential surface of each quartz transducer. The two fiber sections were spliced by 90 to balance differential group delay induced by the intrinsic birefringence. A non-reciprocal faraday rotator was utilized to interrogate the quasi-in-line Sagnac-type reflective interferometer. The principle of the sensor was investigated and the expression of the interference was deduced by using Jones Matrix. The linear relationship between the detectable phase and the voltage to be measured was confirmed by analyzing the mathematic model of sensor head and the scale factor of the sensor was calculated associated with the digital closed-loop model. The experiment shows that the nonlinear error of the scale factor is less than 0.2% when the measured voltage is more than 3 000 V.