The ash fusing characteristics on three sorts of seaweed (a kind of marine biomass) were studied by using thermal microscope, X-ray diffractometer (XRD), ash composition analysis, and simultaneous thermogravimetry/differential thermal analysis. It was presented that there were lots of alkali metals especially K and Na in all seaweed ash samples. XRD analysis shows that the crystalline phase intensities of alkali chlorides reduce with increasing the ashing temperature, due to the evaporization of alkali chlorides. Therefore, the evaporization of alkali chlorides in seaweed biomass should be considered during the thermal conversion. The low ash fusion temperatures of seaweed make the ashing temperatures recommended by both GB and ASTM norms to exceed the limit of temperature to prepare seaweed ash. At a high ashing temperature, Gracilaria cacalia and Sargassum natans will generate some high-melting matters which influence the identification of the ash fusion points. So, it is more exact and referenced that the seaweed biomasses were ashed at a lower temperature (such as 530 degrees C). Besides, the slagging characteristics and fusing characteristics of each sample are quite different. Gracilaria cacalia is the easiest to slag, followed by Sargassum natans. Enteromorpha clathrata is the hardest to slag among them. Gracilaria cacalia and Sargassum natans show a great range from the ash deformation temperature to hemispherical temperature, and Enteromorpha clathrata reveals a small temperature difference between deformation temperature and hemispherical temperature.