The　objective　of　this　paper　is　to　determine　the　influence　of　moveable　mass　on　the　opening　velocity　and　buffering　characteristics　of　fast　vacuum　circuit　breakers　(FVCB)　with　a　gas　pressure　buffer.　A　multi-physics　field　simulation　model　was　built,　which　coupled　the　flow　field,　transient　electromagnetic　field,　mechanical　motion　and　external　circuit.　The　average　velocity　in　the　first　1/3　stroke　(10　mm)　was　defined　as　the　opening　velocity.　Simulation　results　showed　that　with　the　increase　of　moveable　mass　from　4　to　12　kg,　the　opening　velocity　decreased　following　a　negative　exponent　function　from　6.13　to　3.50　m/s;　the　momentum　at　the　end　of　30　mm　full　stroke　increased　following　a　positive　exponent　function　from　12.26　to　21.01　kg.m/s;　and　the　work　done　by　the　buffering　force　decreased　following　quadratic　function　from　443.61　to　203.02　J.　The　relationship　between　buffering　force　and　the　displacement　of　the　moveable　part　was　described　as　a　power　function.　©　2019　IEEE.