The　laser-induced　plume　is　an　essential　physical　phenomenon　during　high　power　fiber　laser　welding.　Its　physical　nature　can　be　investigated　by　diagnosing　its　temperature.　In　order　to　overcome　the　lack　of　sufficient　line　spectra　in　the　temperature　diagnosis,　the　continuous　spectrum　method　(based　on　the　continuous　spectrum　and　Wien＇s　displacement　law)　is　thus　developed　in　this　research.　The　optical　spectra　emitted　by　the　plume　or　plasma　were　detected　by　using　a　SP-2500i　spectrometer　during　bead-on-plate　welding　with　an　IPG　YLS-6000　fiber　laser　or　fiber　laser-arc　hybrid.　The　rationality　of　the　continuous　spectrum　method　was　evaluated　by　comparing　the　plasma　temperatures　diagnosed　by　applying　the　continuous　spectrum　and　line　spectra.　The　results　reveal　that　the　continuous　spectrum　method　used　in　temperature　diagnosis　is　rational.　When　the　line　spectra　cannot　be　detected　due　to　an　increase　in　detected　height,　the　plume　temperature　can　be　still　obtained　by　using　the　continuous　spectrum　method.