This paper presents a new method for measuring AC field strength, with application of electrostriction effect of electrostrictive ceramics and technology of fiber Bragg grating. We use electrostriction effect to design the fiber-optic electric field sensor. This device turns the strain of electrostriction material influenced by electric field into the strain of fiber Bragg grating, then wavelength of feedback changes. We can get the strain of electrostriction material through the comparison between initial wavelength and wavelength of feedback, then measurement of electric field strength can be achieved by detecting the strain of electrostriction material. Experiments indicate that fiber-optic electric field sensor based on electrostriction effect is of effective value for measuring electric field strength, and the response of measuring system to AC electric field is linear. We apply an effective method for the research of measuring electric field strength.