To　study　full　digital　controlled　system　for　CO2　short　circuit,　the　change　of　droplet　volume　in　arcing　period　and　the　behavior　of　liquid　bridge　in　short　circuiting　period　were　analyzed,　and　thus　the　mathematic　model　of　the　dynamic　behavior　of　the　droplet　behavior　and　liquid　bridge　was　built.　A　mathematic　model　was　also　established　to　describe　dynamic　voltage　loading　for　short　circuit　transfer　and　dynamic　processes　of　welding　arc　length.　With　Matlab/Simulink,　the　simulation　model　of　full　digital　controlled　welding　system　model　for　CO2　short　circuit　welding　was　built.　The　experiment　results　validate　the　developed　simulation　model,　and　it　can　be　applied　to　the　research　of　digital　regulator　rules　and　dynamic　behavior　of　CO2　short　circuiting　welding　with　full　digital　controlled　system.