With the rapid popularization of smart phones, online communication has become an important way for people to communicate with each other which result in worsening mobile phone addiction. Previous studies have found that online communication, the most frequently used function, is a significant predictor of mobile phone addiction. However, they have not thoroughly explored the influence mechanism. According to the use-gratification theory and self-determination theory, online communication can help individuals build better interpersonal relationships and obtain more social support (in other words, accumulate social capital), so it can meet individual relationship needs and ultimately enhance the tendency of individual mobile phone addiction. In addition, this effect may be different when communicating with different people. The activation hypothesis of network social contact thinks that online communication can increase the contact between individuals and their friends in real life, practice their ability to get along with friends, and thus promote their interpersonal skills in real life. On the contrary, the alternative hypothesis holds that online communication will occupy the time and opportunity to communicate with real friends, thus reducing the quality of friendship with real friends. Previous studies have shown that online communication with acquaintances conforms to activation hypothesis, while online communication with strangers conforms to substitution hypothesis. Such differences may also exist in mobile phone online communication. Only when communicates with acquaintance, mobile phone online communication can accumulate social capital, meet the needs of the relationship, and finally predict mobile phone addiction. On this basis, this study aims to investigate the relationship between mobile phone online communication and mobile phone addiction, and the mediating role of social capital and relatedness need satisfaction. What’s more, this study further discussed the difference of the mediating function between different communicate. We recruited 3500 students from five middle school in Beijing, Hunan province and Fujian Province. Only 3,268 students completed all four self-report questionnaires (response rate was 93.37%). All scales had good reliability and validity. All data were analyzed by the software of Mplus7.0 and SPSS20.0. After controlling for other use of mobile social functions, the results indicated that: (1) mobile phone online communication had direct and positive prediction on mobile phone addiction no matter communicated with strangers or acquaintances; (2) mobile phone online communication predicted mobile phone addiction through relatedness need satisfaction when communicates with strangers indirectly; (3) mobile phone online communication predicted mobile phone addiction through a multiple mediating path from social capital to relatedness need satisfaction indirectly;(4) mobile phone online communication is a significantly better predictor of mobile phone addiction through the multiple mediating path when communicates with acquaintance than stranger. In conclusion, the present study shows that students are addicted to mobile phones because their relationship needs are satisfied through online communication on mobile phones. Although mobile phone online communication can temporarily bring out positive results (accumulated social capital, satisfied the relationship needs). However, the satisfaction of psychological needs obtained through mobile phones cannot solve the problem in real life (e.g., poor relationship). It can also lead to negative consequences ultimately (e.g., phone addiction).