The utility model relates to an excitation current detection circuit for magnetically controlled reactors. The detection circuit comprises a shunt connected in series to a direct-current excitation circuit of a magnetically controlled reactor, a voltage acquisition circuit, a voltage-to-frequency conversion circuit, an optical fiber transmission circuit, and a frequency-to-voltage conversion circuit. The voltage acquisition circuit acquires an excitation current and converts the excitation circuit into a voltage signal of certain amplitude, and then the voltage-to-frequency conversion circuit converts the voltage signal into a frequency signal. Optical transmission of the frequency signal is realized through the optical fiber transmission circuit. Then, the frequency-to-voltage conversion circuit converts the frequency signal into a bipolar voltage signal, and the voltage signal finally obtained is a magnetically controlled reactor excitation current signal after conversion and processing. The detection circuit of the utility model is used for detecting and monitoring the excitation circuit of a magnetically controlled reactor. Through conversion of electrical signal-optical signal-electrical signal, electrical isolation between the detection circuit and a high-voltage power grid is realized.