The post-hole convolutes (PHCs) are used in pulsed high-power generators to add the output currents of the magnetically insulated transmission lines (MITLs) and deliver the combined current to a single MITL. Then the single MITL delivers the combined current to the load. Magnetic insulation of electron flow is lost near the post-hole convolute (PHC) in the high-power generator. Although cathode plasma and anode ions are widely considered as the factors of the magnetic insulation collapse, there are some other factors that are needed to study. In this paper, the cathode negative ions are considered in the PIC simulation of a single-hole PHC. In this work, we examine the evolution and dynamics of the negative ions in the PHC. The simulation results demonstrate that there are no current losses while the cathode emits only electrons, little current losses (10 kA out of a total current of 900 kA) while the cathode emits plasma including electrons and ions, and obvious current losses (20 kA out of a total current of 900 kA) while the cathode emits plasma including the electrons, ions and negative ions. The results also indicate that the velocity of the negative ions is about 10 cm/μs, larger than that of the cathode plasma including the electrons and the ions. All results suggest that the cathode negative ions can play an important role in the magnetic insulation collapse, and should be considered carefully in experiment.