In Arctic winter and spring 2020, the stratospheric temperatures were exceptionally low for a long time period and the polar vortex was very stable. As a consequence, significant ozone depletion occurred in Northern polar regions in spring 2020. Here, we present simulations by the Chemical Lagrangian Model of the Stratosphere (CLaMS) that addresses the development of chlorine compounds and ozone in the polar stratosphere in 2020. The simulation is able to reproduce relevant observations which is shown by comparisons with MLS, ACE-FTS and OMI data. Although the concentration of chlorine and bromine compounds in the polar stratosphere has decreased by more than 10% compared to the peak values around the year 2000, the meteorological conditions in winter and spring 2020 caused an unprecedented ozone depletion. The simulated lowest ozone mixing ratio was around 0.05 ppmv and the ozone depletion in the vortex core in the lower stratosphere reached 133 Dobson Units, which is more than the loss in the years 2011 and 2016 that had the largest Arctic ozone depletion so far.