To determine the ash characteristics, catalysts transformation and the sintering mechanism during catalytic coal gasification, the investigation of the effect of ashing temperature, K2CO3 and CaO catalysts addition on the ash sintering behavior of bituminous coal was conducted under catalytic gasification conditions. The ash sintering temperature was determined at 3.5 MPa using a pressure-drop sintering device with H2O, H-2, CO inlets. The ash morphology was analyzed using a scanning electron microscope-energy dispersive X-ray spectrometer (SEM-EDS). An X-ray diffractometer (XRD) in combination with FactSage were used to predict the reactions occurring between minerals as well as the mineral transformation and slag formation. The results showed that different ashing temperature affected both ash transformation reactions and volatilization amount of added catalyst, which influenced the ash fusion temperatures and the sintering temperature. The ash-550 degrees C was chosen to investigate the sintering behavior. The ash fusion temperatures and the sintering temperature both dropped markedly to a limit value as the concentration of K2CO3 increased and increased again. Besides, the morphology changes from SEM-EDS validated the trend of the sintering temperature, and the molten degree of the tested C-K-S sample was more serious. The sintering happened at several hundred Celsius below the initial deformation temperature; it is more accurate using FactSage to predict slagging compared with the fusion temperatures in catalytic coal gasification process. Furthermore, the FactSage and XRD results revealed that the existence of massive kaliophilite was the main cause of the greatly decreased sintering temperature, the K bearing aluminosilicate could react with Fe, Ca-containing aluminosilicates to form eutectic mixtures, which led to a dramatic drop of the sintering temperature. In addition, the effect of the CaO addition on the slagging property depends on the amount of CaO, reaction condition (pressure, reaction atmosphere) and raw coal property, especially to high iron coal. The existing Fe(II) components easily reacted with the Ca-bearing feldspar minerals to produce low-temperature eutectics and formed the liquid phases, which accelerated the sintering and agglomeration. At last, from the view of K, Ca-containing catalysts transformation using XRD and FactSage calculation, the additional catalyst partly transformed into silica, aluminosilicates and entered into cofusions during sintering, which led to the decreasing catalytic activity and increasing difficulty of catalyst recovery in catalytic coal gasification process.