Taking　advantage　of　the　symmetry　in　the　distribution　of　the　absorbed　pump　power　density　of　a　diode　side　pumped　thin　disk　laser,　the　pump　uniformity　could　be　described　quantitively　with　a　novel　method　that　took　values　on　key　points　of　absorbed　pump　power　density　distribution　into　consideration.　A　pump　performance　index　that　considered　both　pump　uniformity　and　absorption　efficiency　was　introduced.　Pump　performance　index　within　certain　ranges　of　diode　array　distance,　round　thin　disk　radius　and　absorption　coefficient　were　compared.　Results　of　calculation　show　that　the　performance　index　reaches　an　optimized　minimum　of　0.1110　with　the　round　thin　disk　radius　of　13　mm,　the　diode　array　distance　of　37　mm,　and　the　absorption　coefficient　of　0.10　mm-1.　A　performance　index　near　to　the　optimized　value　could　be　obtained　with　the　parameters　experimentally　used.　Tentative　experiment　measured　the　phase　distortion　on　the　thin　disk　caused　by　thermal　effects.　The　computed　phase　distortion　matches　with　experimentally　measured　phase　distortion.　Pump　performance　index　and　related　calculation　results　presented　in　this　paper　presents　a　useful　theoretical　reference　for　research　of　diode　side　pumped　composite　thin　disk　lasers.