Q-switching　in　a　mode-locked　laser　not　only　makes　the　amplitude　of　every　single　output　pulse　unequal,　but　also　limits　the　time　width　and　peak　power　of　output　pulses.　This　paper　investigates　the　Q-switching　in　a　self-mode-locked　semiconductor　disk　laser　numerically　and　experimentally.　By　using　the　delay　differential　equations　for　passively　mode-locking,　conditions　of　Q-switching　in　a　self-mode-locked　semiconductor　disk　laser　are　numerically　analyzed　for　the　first　time.　Meanwhile,　based　on　the　experimental　results,　the　causes　of　Q-switching　tendency　including　the　change　of　nonlinear　refractive　index　and　the　change　of　soft　aperture,　are　also　discussed.　Some　possible　measures　to　suppress　Q-switching　instability,　i.e.,　to　obtain　stable　continuous-wave　mode-locking　in　a　self-mode-locked　semiconductor　disk　laser　are　proposed.