Abstract: A small size 3D fiber-optic electric field sensor for measurement of electromagnetic pulse (EMP) in small volumes is developed. The influence of the sensor structure on the tested field is analyzed by CST2011. Meanwhile, the disturbance from shielding enclosure can be effectively reduced by simulating its embedded and spherical structure. The sensor interelectrode coupling and the three-dimensional field error of the synthesized field are discussed. Design parameters are defined based on the analysis results. A conformal sensor structure is utilized to act both as the dipole antenna and the shielding enclosure. The shielding efficiency of the external electric field can reach 60 dB. Experiments in the bounded-wave EMP simulator show the sensor can meet the requirements for three-dimensional electric field measurement and has smaller perturbation to the measured field. The interelectrode coupling coefficient is less than 6% and the error of the synthesized field is less than 5%. The balanced structure of the sensor makes it suitable for EMP measurement in small volumes and in free field.