The invention belongs to a high and low voltage isolating circuit, which comprises two voltage input terminals and two voltage output terminals, and is mainly characterized in that: a nonlinear element is respectively parallelly connected between the two voltage input terminals and the two voltage output terminals, the two voltage input terminals are respectively connected with two photoelectric couplers, and the two photoelectric couplers are respectively connected with the two voltage output terminals. The high and low voltage isolating circuit effectively solves the safety problems of human bodies and instruments in a measuring end which are caused by intermittent excess voltage, chopped wave excess voltage and excess voltage situation that is generated by the sudden raising of earth point potential in the alternate and direct current withstand voltage testing process, is characterized by quick response speed, prompt protection, simple use and convenient transplantation, and can bewidely applied into a measuring loop of the alternate and direct current withstand voltage testing and other measuring devices that need electric protection and isolation.