The fouling problem often occurs in multi-effect evaporators.These affect the thermal efficiency and normal operation of the device.In order to identify the formation causes and composition of fouling in the evaporator,a series of methods,including X-ray diffraction（XRD）,plasma emission spectroscopy（ICP）,scanning electron microscopy（SEM-EDS）,polarized light microscopy,Raman spectroscopy and ion chromatography were used to characterize and analyze the unknown fouling on the ammonium chloride multi-effect evaporator of Zhejiang Dayang Biotechnology Group Co.,Ltd..The results showed that the main component of the scale is a mixture of iron phosphate,alumina and organic matter.In addition,the cause and mechanism of the scale formation was discussed and the corresponding solutions for reducing the fouling in the evaporator was also proposed.