The invention discloses an SPD (surge protective device) back-up protector based on electromagnetic differential motion, and relates to a thunder and lightning overvoltage protection device. The SPD back-up protector comprises an SPD (20) and a screw press connecting wire terminal (30), and is provided with a protection unit (10), wherein the protection unit (10), the screw press connecting wire terminal (30) and the SPD (20) are sequentially connected; the protection unit (10) comprises a contact switch (11), an electromagnetic differential motion industrial frequency overcurrent detector (12), an overcurrent triggering mechanism (13), a manual switching-on handle (14) and a fault indicator (15). The SPD back-up protector provided by the invention has the advantages that the screw press connecting wire terminal of a conventional small-size short circuit device is used; the wire connection is convenient; the electromagnetic differential motion industrial frequency overcurrent detector is used, and has the performance that the triggering can be realized when the current is smaller than the 1A industrial frequency current, and the non-triggering can be realized under the effect of high-amplitude thunder and lightning flash surging; under the effect of the thunder and lightning flash surging, low residue pressure is shown; the excellent thunder and lightning protection performance is realized; the use safety of a system is guaranteed; the SPD back-up protector is suitable for the backup protection of various kinds of SPDs.