With the rapid rise and popularity among people all over the world, social network sites have become an important mean and platform of interpersonal interaction, and the impact of social network sites’ use has also become a focus of attention of relevant research scholars. Social network sites, aiming at encouraging people to build and maintain a network of friends, provides a necessarily supplementary way to interact with others, which has an important effect on individuals’ psychological and social adaption. Researches has revealed that: the use of social networking sites could increase individual’s social capital, reduce individual’s loneliness, and promote life satisfaction and subjective well-being. But some researchers have also found inconsistent results - the use of social networking sites was significantly and negatively correlated with self-esteem and happiness, and significantly and positively correlated with depression. For these inconsistent results , researchers have pointed that the impact of social network sites’ use on individual was affected by other variables. At the same time, current researches are mainly focusing on the impact of social network sites’ use on individual’s psychosocial adaptation. But the core development task of adolescents, the formation and development of self, has not got enough attention. Previous studies have pointed that, individuals could construct themselves through self-presentation and self exploration on social network sites because social network sites could provide teenagers with an ideal self-exploration space. While Valkenburg and Peter (2011) outlined the self-concept fragmentation hypothesis: the opportunities that internet afforded individuals to interact with many different people in diverse online environments - in some cases adopting a different　personality in each one - undermined their ability to coordinate the multiple facets of themselves into a coherent whole, and this hypothesis has been confirmed by several researches. On this basis, the aim of this study was to investigate the effect of social network site on self-concept clarity, and further explored the effect of social comparison orientation. And a survey had been conducted to examine the relationship among social network site use, social comparison orientation and self-concept clarity of middle and high school students. The participants were 697 middle school students; and three questionnaires were used in this study: the Scale for Social Network Site Use Intensity, Iowa－Netherlands Comparison Orientation Measure, and Self-concept Clarity Scale. The results indicated that: (1) Social network site use was positively correlated with social comparison orientation, but social network site use and social comparison orientation were negatively correlated with self-concept clarity. (2) The mediating effect of social comparison orientation was significant in social network site use’s effect on self-concept clarity. That’s to say, not only could social network site use significantly predicted self-concept clarity, but it could also affect self-concept clarity through the mediating role of social comparison orientation. This indicated that, though the social network site use has a positive effect on individuals’ psychological and social adaption, it also has a negative impact on individual’ self-development. This seemingly contradictory result reflects the different aspects of social networking site’s effect on individuals, and suggest we should attaches great importance to the variables affecting social network sites’ effect on individuals, and also guide teenagers to view friends’ information in social network sites objectively and reasonably.