The　theoretical　analysis　and　experimental　study　of　the　instability　phenomenon　of　a　Nd:　YAG　Q-switched　short　pulse　laser　was　demonstrated.　During　the　operation　of　the　short　pulse　laser,　there　were　many　factors　that　can　cause　the　instability　phenomenon.　During　the　experiment　it　was　found　that　the　interference　pulse　which　can　enter　into　the　cavity　though　the　output　mirror　can　cause　the　instability　of　the　short　pulse　laser.　Based　on　the　basis　of　thory,　the　influence　of　external　interference　light　on　the　output　light　characteristics　of　short　pulse　Q　-switched　laser　was　simulated　and　verified　by　experiments.The　characteristics　of　energy　and　pulse　width　were　discussed　respectively.　The　results　show　that　the　greater　the　energy　of　interference　pulse　is,　the　more　serious　the　instability　phenomenon　is.　When　the　interference　pulse　energy　is　60　mJ,　the　loss　of　output　energy　of　the　short　pulse　laser　can　be　more　than　15%.　The　pulse　width　is　1.15　times　of　the　original,　the　instability　phenomenon　is　also　more　serious　at　the　pump　end　of　the　short　pulse　Q-switched　later.　When　the　input　energy　is　60　mJ　the　pulse　width　is　10　ns,　the　loss　of　output　energy　of　the　short　pulse　Q-switched　laser　reaches　80%　of　the　original.　©　2018,　Editorial　Board　of　Journal　of　Infrared　and　Laser　Engineering.　All　right　reserved.