Surfactants exist widely in natural water such as sea water and river water, which may be used as cooling water source for nuclear power plants. Although its concentration is much lower than other impurities, it has a significant effect on boiling heat transfer. A small amount of surfactant can greatly enhance the heat transfer coefficient but its mechanism is not fully understood. Moreover, there is great disagreement in the research community about its effect on the boiling critical heat flux. In this study, we carried out a series of visual pool and flow boiling experiment to study the mechanism of surfactant action on boiling heat transfer through bubble dynamic behaviors. It was found that the change in boiling performance was mainly due to the non-coalescence of growing bubbles on the wall, rather than the surface tension changes of the solution which was seen as the main reason in the past. The conclusion of this study is instructive for the research on the use of natural water as cooling water for nuclear power plants.