Pneumatic　micro-droplet　ejection　is　a　printing　technique　that　has　potential　applications　in　many　fields,　especially　in　the　field　of　bio-printing.　The　ejection　is　via　a　solenoid　valve　being　briefly　turned　on,　so　that　high　pressure　gas　enters　the　liquid　reservoir,　forming　a　gas　pressure　pulse,　forcing　the　liquid　out　through　a　tiny　nozzle　to　form　a　micro-droplet.　For　bio-printing　applications,　the　bio-inks　are　typically　non-standard.　The　difficulties　are　not　only　that　the　initial　working　parameters　are　difficult　to　set,　but　also　the　working　conditions　change　over　time　in　many　cases.　In　order　to　maintain　a　stable　single-drop　ejection　state,　a　machine　vision　based　ejection　monitoring　was　designed　to　obtain　the　number,　positions　and　sizes　of　the　droplets　for　each　ejection,　and　a　feedback　control　is　realized　by　adjusting　the　＂ON＂　time　of　the　solenoid　valve　or　the　gas　pressure　at　the　front　end　of　the　solenoid　valve.