The invention belongs to the technical field of electrical power systems, and discloses a cold-insulating high-temperature superconducting cable superconductivity losing detecting circuit and a detecting method of the cold-insulating high-temperature superconducting cable superconductivity losing detecting circuit. The superconductivity loosing detecting circuit comprises a first current sensor, a second current sensor, a first optical coupling isolation circuit, a second optical coupling isolation circuit, an analog division circuit and a comparison circuit. The first current sensor and the first optical coupling isolation circuit are connected to form a first collecting and isolating circuit. The second current sensor and the second optical coupling isolation circuit are connected to form a second collecting and isolating circuit. The first collecting and isolating circuit and the second collecting and isolating circuit are connected with the analog division circuit and the comparison circuit respectively in sequence. Through the detecting circuit and the detecting method, the superconductivity losing failure of a superconducting cable can be fast detected, and detecting accuracy is high.