A nearly all-superconducting power system is considered as a promising path to realize Turboelectric distributed propulsion (TeDP) Aircraft. The options of power switches are rather limited and only cryogenic power electronics are into the scope so far. A mechanical switch using LN2 as an arc extinguishing medium (LN2 switch) can be a promising alternative that has lower conduction loss. However, basic research on current interruption in LN2 is still lacking, especially the effect of magnetic fields. The objective of this paper is to understand the effect of the magnetic field on DC current interruption by LN2. Interruptions under different magnetic fields are tested. A two-stage arcing process is observed under the magnetic field. Stage 1 is similar to that in the air while stage 2 is distinguished as interactions between vaporized LN2 and arc. 600 V DC/1 kA is interrupted by LN2 under 62.5 mT successfully. The interruption current is 8 times higher than that of without magnetic field applied. The magnetic field strengthens arc cooling by enhancing arc motion in LN2. Adding no burden to refrigeration, an LN2 switch can be a promising application in HTS electrical system.