In the survey of STD abroad, we pay special attention to the number of sexual partners, the number of sexual acts, and the way of sexual acts, so we can see that the diversity of sexual acts brings new issues to the prevention of STD. According to Japanese literature, the percentage of STDs contracted by oral sex was 24% in 1981, and has increased year by year since then, reaching 70-76% this year. In the oral microecology, there is a biological balance between microorganisms, host and environment. The oral microecological system is composed of a complex of oral microorganisms and the host oral cavity. “Oral sex” can cause the following diseases: 1, acute and chronic urethritis: oral sex can be oral microbiota, with saliva migrated to the male urethral opening (inside), coronal groove and foreskin plate, bacteria or mycobacteria and then proliferate, invading the urethra upstream caused by infection. 2, bacterial vaginitis: oral sex after intercourse, will have been left on the penis of oral bacteria through sexual intercourse transplanted into the vagina, can make the vaginal biological barrier is destroyed, causing an imbalance in the vaginal flora, bacterial vaginitis occurs. 3, oropharyngitis, periodontitis: if oral sex after intercourse, the same vaginal bacteria into the oral cavity, will also destroy the ecological balance of the oral cavity, oral and pharyngeal disorders, and even acute or chronic periodontitis.