The invention belongs to a high and low voltage isolating circuit, comprises two voltage input ends and two voltage output ends, and has the following essential technical characteristics: a nonlinear element is respectively parallel connected between the two voltage input ends and the two voltage output ends, the two voltage input ends are respectively connected with two photoelectric couplers, and the two photoelectric couplers are respectively connected with the two voltage output ends. 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 be widely applied into a measuring loop of the alternate and direct current withstand voltage testing and other measuring devices that need electric protection and isolation.