This report focuses on contemporary unmarried youth’s idea on sex and sexual behavior，the related empirical data comes from “National Youth Survey” and “Chinese General Social Survey”. According to the findings，the youth holds the attitude of hesitation over premarital sexual behavior on the whole；themen，high-educated and urban youth hold more tolerant attitude over premarital sexual behavior；for the unmarried youth who have higher demand for marriage quality and emphasize having a common goal，they tend to show more tolerant attitude over premarital sexual behavior and premarital cohabitation；for the unmarried youth who are men and have rural household registration，the proportion of sexual behavior is higher；the affection is the important factor to influence whether contemporary youth have premarital sexual behavior or not，while the constraint effect of institution norms on cohabitation or marriage is decreasing. The study also finds that the unmarried youth have insufficient safety consciousness and safety knowledge regarding sexual behavior；for the rural youth especially the youth having the experience of left-behind children，they tend to face rising risks in unsafe sexual behavior. Based on these findings，we try to put forward some related policy suggestions：the school，family and society should collaborate together to construct the support system for sexual education and reproductive health service，especially strengthen the protection and care towards vulnerable groups including rural youth and left-behind children.