Micro-droplet　ejection　is　a　liquid　dispensing　technology　that　has　potential　applications　in　many　fields.　Specifically,　pneumatic　ejection　is　actuated　by　a　solenoid　valve,　which　is　set　to　＇conduction＇　state　for　a　brief　period　of　time　Δt.　High　pressure　gas　of　P　0　enters　the　liquid　reservoir,　then　releases　through　a　venting　tube,　creating　a　oscillating　pressure　waveform　P(t),　forcing　the　liquid　out　through　a　tiny　nozzle　to　form　a　micro-droplet.　For　each　actuation,　P(t)　is　acquired　by　a　high-speed　pressure　sensor,　and　the　ejection　state　is　obtained　by　high-speed　photography　and　image　processing　methods.　Some　issues　for　the　design　of　pneumatic　micro-droplet　ejector　are　discussed.　For　simulation　of　P(t),　it　is　proposed　within　an　electro-acoustic　analogy　picture　that　the　acoustic　resistance　of　the　venting　tube　is　mainly　due　to　viscous　effect　and　may　vary　with　time　during　the　whole　ejection　process.　Based　on　this　assumption,　the　calculated　P(t)　is　more　consistent　with　the　actual　measurement.　Experimentally,　the　droplet　ejection　process　for　different　length　of　venting　tube　is　studied.　With　P　0　and　Δt　set,　by　increasing　the　venting　tube　length　L,　both　the　peak　value　P　MAX1　and　duration　of　the　first　positive　pressure　period　increase,　and　more　droplets　are　ejected　from　a　single　actuation.　By　setting　different　P　0,　P　MAX1　for　different　L　is　tuned　to　an　identical　and　appropriate　value,　so　that　single　droplet　is　ejected　due　to　the　first　positive　pressure　period.　However,　with　the　increase　of　L,　the　peak　value　of　the　second　positive　pressure　period　P　MAX2　increases.　There　is　a　certain　probability　that　another　droplet　is　ejected.　It　is　realized　that　the　increase　of　L　can　reduce　gas　consumption,　but　the　multiple　ejection　is　a　drawback　that　should　be　considered　in　the　design　of　pneumatic　ejection　system.　©　2021　IOP　Publishing　Ltd.