With the rising concerns on environmental pollution and emission reduction, plastic waste disposal draws great attention from governors and scientists. Instead of landfilling and incineration, chemical treatments are considered less polluting and resources recovering. Hydrothermal processing is considered a prospective technology for this issue. To regulate the conversion process and improve efficiency, a key step is to know the dominant stages and clarify the controlled part of the reaction. However, there is very limited number of researches on the visualized exhibition of this process. In this paper, a hydrothermal process visualization measure is adopted, to intuitionally demonstrate the whole conversion process. Polyoxymethylene (POM) spheres are employed as the feedstock in this experiment. A fixed-focus camera records the plastic sphere particle undergoing three stages of heated, swelling and dissolution, under hydrothermal condition. At the heated stage, the solid sphere becomes transparent, then during the swelling stage, the transparent sphere expands to around 130 % of the original size (sectional area), finally the expanded sphere is dissolved into fragments dispersing in the water. During the hydrothermal conversion, the carbon component of the plastic sphere immigrated into the aqueous phase, carbon recovery rate of the process reached as high as 90 %. The visualized results well demonstrate the physical and chemical transition of hydrothermal process, which would be a promising direction for future study.