Current loss without an obvious impedance collapse in the magnetically insulated coaxial diode (MICD) is studied through experiment and particle-in-cell (PIC) simulation when the guiding magnetic field is strong enough.Cathode negative ions are clarified to be the predominant reason for it.Theoretical analysis and simulation both indicate that the velocity of the negative ion reaches up to 1 cm/ns due to the space potential between the anode and cathode gap (A-C gap).Accordingly,instead of the reverse current loss and the parasitic current loss,the negative ion loss appears during the whole pulse.The negative ion current loss is determined by its ionization production rate.It increases with diode voltage increasing.The smaller space charge effect caused by the beam thickening and the weaker radial restriction both promote the negative ion production under a lower magnetic field.Therefore,as the magnetic field increases,the current loss gradually decreases until the beam thickening nearly stops.