We design an isotropic electric-field sensing system using an optical probe.The optical probe consists of three electric-field sensors based on a Mach-Zehnder interferometer with the structure of segmented electrodes,which are perpendicular to each other.The measured results show that the ±5 dB baseband can reach 3 GHz,the linear dynamics range is 70dB,and the minimum detectable electric field is lower than 90 dBμV/m(the spectrum analyser resolution is 100 Hz).The directional characteristics are almost isotropic within deviations of±1.5 dB.Therefore this sensing system can be used in the electromagnetic compatibility measurements.