Three-dimensional ensemble Monte Carlo model is employed using C programming language to investigate the impact of different cathode aperture sizes to micro-hollow cathode discharges. Results show that in argon under constant pressure and voltage, when an external magnetic field is added in a micro-hollow cathode discharge, the distribution of electron density has not change much in cathode, and electron drift range reduces in the axial and lateral direction, and electron energy becomes lower. But the applied magnetic field has a less obvious effect on high-pressure micro-hollow cathode discharges than low-pressure discharges on the whole. The research results have a certain guiding significance on the mechanism of the micro-hollow cathode discharge plasma.