Tunnel　cascaded　and　coupled　multi-active　regions　laser　diodes　are　novel　high　power　laser　diodes.　This　kind　of　laser　diodes　can　achieve　high　output　power　at　relatively　low　current　density　and　overcome　the　main　hindrance　of　the　normal　high　power　semiconductor　lasers:　catastrophic　optical　damage　(COD)　by　increase　the　size　of　facula.　Transient　thermal　property　of　these　laser　diodes　has　been　calculated　by　using　finite　element　method　(FEM).　Three　kinds　of　laser　diode　structures,　one　active　region,　two　active　regions　with　one　tunnel　junction　and　three　active　regions　with　two　tunnel　junctions,　are　simulated.　The　calculated　results　are　in　agreement　with　the　measured　data.　The　result　indicates　that　for　tunnel　cascaded　and　coupled　multi-active　regions　laser　diodes,　temperature　rising　of　the　active　region　near　the　substrate　is　a　little　higher　than　that　near　heat　sink.　With　active　region　number　increasing,　the　temperature　of　the　laser　diodes　rises　but　multi-active　regions　were　fabricated　on　the　uniform　substrate,　its　thermal　resistance　is　still　smaller　than　that　of　series　with　the　same　number　normal　laser　diodes.