Mulberry powdery mildew is a common disease in mulberry fields. The symptoms appears mainly on the middle and lower leaves. In the late stage of disease, the leaves turned yellow, emerged brown spots and withered eventually, which seriously affects the yield and quality of mulberry leaves. The pathogen of mulberry powdery mildew is the mulberry ball needle shell powdery mildew fungus belonging to the Ascomycota phylum. In this study, the morphological characteristics of mulberry leaves infected with pathogens was monitored, which was divided into three stages namely initial stage of infection, middle stage of infection and ultimate stage of infection. Absolute quantitative analysis was used to determine the content changes of pathogens at different stages of infection, and the content changes of main substances(like chlorophyll, total sugar, vitamin C, POD etc.) in mulberry leaves at different stages of infection were also determined. Real time quantitative PCR was used to analyze the expression level of a series of MLO(mildew resistance locus O) genes. The results showed that the concentration of the pathogen increased after infected, the lesion site expanded and deepened. Besides, the contents of chlorophyll, total sugar,vitamin C, POD and SOD decreased, the contents of soluble protein, CAT and water increased and then decreased, while the content of proline increased. Sixteen MLO genes were detected, among which five genes were expressed and the expression levels of MnMLO6A and MnMLO2 were down-regulated while the expression levels of MnMLO13, MnMLO1A and MnMLO4B were up-regulated. Among these MLOs,MnMLO2 attracted more attentions for its linear relationship between expression level(significant down-regulated) and infection degree. These results provide a basis for exploring the occurrence regularity,effect on mulberry physiological process and the pathogenic mechanism of mulberry powdery mildew, and also provide a certain guiding role in the prevention and control of mulberry powdery mildew.