The precipitation, and fusion,of mineral,crystals in coal slag significantly affect the Viscosity. In this paper, the crystalliation process of two iron-rich coal slags under continuous cooling was studied. High-temperature stage microscopy (HTSM) was applied to observe and record the dynamic crystallization process. Crystal morphology was measured and analyzed, and the results showed that the high cooling rates increased the crystal morphologies and sizes Obviously. The whole crystallization process was close to the analysis results of the viscosity-temperature curve. Scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS) detected the microstructures and element compositions of the crystal, respectively. The results revealed that the crystals from slag B were mainly gehlenite in accordance with the results of X-ray diffraction (XRD). The element analysis of crystals from slag A was different from the detection results of XRD for the special crystal-structure of the iron element.