In view of the phenomenon that the catalyst on the top of hydrofining reactor was covered with scale when the diesel hydrocracking unit in a refinery was shut down,accompanied by replacing catalyst,the sampling analysis of guard catalyst was conducted at the same time.The X-ray fluorescence spectrometer（XRF）,scanning electron microscope（SEM）,inductively coupled plasma spectrometer（ICP-OES） and other analysis were carried out to analyze guard catalyst after toluene extraction.The results showed that the main impurities depositing on guard catalyst were inorganic componentes containing Fe,P,Si,Ca and Ti elements,and impurities containing Fe,P,and Si elements were mostly deposited on the surface of guard catalyst,while impurities containing K,Ca and other elements were uniformly deposited on the guard catalyst,without other phase structures on the guard catalyst.In this regard,it is suggested that the refinery should strictly control the carrying of impurities containing P,Si and other elements in the raw oil,and on the other hand it is necessary to load some silicon trap catalyst to optimize the grading and loading of guard catalyst,so as to realize the smooth operation of the device.