The influence of the quartz, which acts as the sensing element of the fiber optic voltage sensor, on the perfermanee of the sensor is studied. Both theoritieal analysis and experimental results show that the fluctuation of the temperature brings about the undulation of the characteristic of the sensing crystal and the variation of the stress applied to the crystal, causing the consequence that the long-term stability of the sensor can not meet the requirement of measurfing aeeurey of 1%·