The measurement of surface charge for insulating dielectrics is critical to estimate insulation strength of electrical equipment under electric field. However, the electrostatic probe with power supply for measuring charge is susceptible to electromagnetic interference from electrical equipment. Thus, the investigation of an electrostatic sensor without power supply for monitoring the dynamic evacuation behavior of the deposited charge on the surface of the insulating dielectrics is of great significance. Here, a passive optical fiber electrostatic sensor based on the principle of Fabry-Perot interference is developed to measure the surface potential of the insulating dielectrics. The sensitivity to quantify the electrostatic charge is explored in terms of the phase shift in interference spectrum and the linear correlation is over 0.99. Meanwhile, the influence of surface morphology on the surface potential could be also identified by the sensor proposed in this paper. Furthermore, measured surface potential exhibits a negative quadratic trend with the increase of the surface roughness of the insulating dielectrics. These results reflect that the sensor has the merits of passive monitoring, bipolar electrostatic detection, and excellent ability of anti-electromagnetic interference, which is expecting to be developed to a novel tool to monitor the electrostatic charge in the fields of smart electrical equipment, fuel transportation, aerospace and integrate circuit etc.