Abstract: Comparison of the composition,physicochemical properties and structure changes of the mustard dietary fiber were analyzed before and after cellulase modification in the different temperature and p H. The results showed that the soluble dietary fiber content after modified was increased by 80.02% and the insoluble dietary fiber content after modified was decreased by 9.76%. The water holding capability,swelling capability,and nitrite in the modified dietary fiber were improved significantly(p <0.01). The cation exchange capacity between modified and rude fiber had significant differences(p <0.01),and it reached maximum which was0.657mmol/g at 60℃. Absorption cholesterol capability of the fiber modified at p H7 was more power than that at p H2. By observing the ultra at structure of dietary fiber before and after modification,the modified fiber had smaller particle size,looser morphology and structure,larger cavity and crack,the main components and their chemical structures were not affected.