In　an　optically　pumped　semiconductor　disc　laser,　which　is　always　with　a　multiple　layer　active　region　and　a　large　spot　size,　the　spontaneous　emission　of　quantum　wells　will　be　trapped　by　the　waveguide　caused　by　the　multiple　layer,　travel　a　long　excited　distance　relative　to　the　surface-emitting　mode　and　be　subsequently　amplified.　Here　we　demonstrate　a　new　mechanism　for　the　amplified　spontaneous　emission　(ASE):　the　etalon　effect　of　the　sub-cavity　formed　by　the　bottom　distributed　Bragg　reflector　and　the　top　air-semiconductor　interface.　The　spectral　characteristics　of　this　ASE　including　its　temperature　dependence,　pump　power　dependence　and　observing　angle　dependence　are　experimentally　studied,　and　the　results　are　in　good　agreement　with　the　theoretical　model.