The invention discloses an electric field measurement based internal defect detector for insulators. The detector comprises a portable device of a live insulator detector and a data receiving terminal machine, and the portable device of the live insulator detector is connected with the data receiving terminal machine in a wireless ad hoc network communication manner; and the portable device of the live insulator detector comprises a single-chip microcomputer, an electric field sensor, a photoelectric sensor and a data collection unit. According to the internal defect detector for the insulators, vertical electric field distribution of an insulator chain is extracted, an internal defect of the insulators is determined according to the vertical electric field distribution, data is transmitted to a computer of a ground worker via wireless network, and is displayed in manners as a curve, internal defects of the composite insulators in a high tension transmission line can be detected in a live-line manner, zero insulators can be identified, whether certain insulator has an internal defect fault can be displayed visually, convenience is provided for searching fault insulators from the transmission line, and the power supply reliability is improved.