I will give a presentation at the European Astronomical Society Annual Meeting 2022. The presentation is about our recent X-ray study of RCW 86 (ApJ, under review). We investigated the shock velocity, thermal & non-thermal X-ray properties of the southwestern region of RCW 86. Our findings suggest that ambient density controls the particle acceleration conditions: low-density regions show low synchrotron fluxes & high maximum energies. We think this can be explained by the shock-cloud interaction theories.