The thermal stability and photoluminescence degradation of Mn2+ in fluorescent lamps used BaMgAl10O17:Eu2+,Mn2+ phosphor are investigated. The detection of impurity containing manganese with high valence demonstrates the oxidation of Mn2+ during baking process at 700°C and it partly leads to emission degradation of Mn2+. The emission of Mn2+ mainly originates from the energy transfer of Eu2+→Mn2+. The oxidation of Eu2+ decrease the amount of Eu2+ centers and thus finally causes the degradation of Mn2+. Some traps are revealed to be induced during baking process and it further weakens energy transfer efficiency of Eu2+→Mn2+ and decreases the emission of Mn2+.