We calculate the electromagnetic (EM) decay widths of the B ± c (2S) meson, which is observed recently by the ATLAS Collaboration. The main EM decay channels of this particle are 1 3 S 1 γ and 1P γ, which, in literature, are estimated to have the branching ratio of about 1/10. In this work, we get the partial decay widths: Γ(2 1 S 0 → 1 3 S 1 γ) = 0.192 keV, Γ(2 1 S 0 → 1P 1 γ) = 2.24 keV and Γ(2 1 S 0 → 1P ′ 1 γ) = 11.4 keV. In the calculation, the instantaneous approximated Bethe-Salpeter method is used. For the P -wave B c mesons, the wave functions are given by mixing the 3 P 1 and 1 P 1 states. Within the Mandelstam formalism, the decay amplitude is given, which includes the relativistic corrections.