The result of this study highlights the profound effect of collective heating on the possible parameter space for CW photothermal catalysis. Under CW laser irradiation, collective heating enabled high temperatures (e.g., melting temperature of TiO2) to be reached under moderate laser power. The temperature of the plasmonic Au nanoparticle rose to the maximum temperature in a short time (~ 0.16 μs). Within this brief period, the surrounding TiO2 experienced the same high temperature as the Au NPs due to collective heating. Our work shows experimental evidence that photothermal heating from CW illumination could introduce phase transition of the catalyst and reach as high as a couple of thousand Kevin in photoreaction. The results suggest that the photothermal effect could be a significant mechanism in the plasmon-assisted photocatalytic reactions.