Based　on　the　fluorescence　intensity　ratio　technique,　the　temperature　distribution　inside　an　Er:Yb:Lu2Si2O7　crystal　was　measured　in　real　time　when　a　1.55　mu　m　laser　was　operating.　The　polarized　green　up-conversion　fluorescence　in　500-580　nm　of　the　crystal　generated　simultaneously　under　laser　operation　was　used　as　the　temperature　probe.　At　a　pump　beam　waist　diameter　of　400　mu　m,　the　highest　temperature　inside　the　crystal　reached　507　K　when　the　incident　pump　power　and　output　mirror　transmission　were　11.13　W　and　1.3%,　respectively.　The　experimental　result　was　compared　with　the　theoretical　temperature　distribution　inside　the　crystal　simulated　by　the　finite　element　analysis　method,　and　the　difference　between　them　was　analyzed.