Smart-phone is an important channel of interpersonal communication. As the rapid development of communication technology，its function is becoming more and more powerful. Meanwhile，people are much more dependent on it. This study aims at investigating how long do undergraduates spend on their smart-phones everyday，as well as its feature and pattern. The design of this study involved an experience sampling methodology（ESM）；time of smart-phone use everyday from 105 undergraduates were recorded and collected by an APP on their phones for a whole week，resulting in a total of 735 observations. Results showed that the average total time of undergraduates spending on smart-phone is 5.29（SD=2.53）hours everyday，38% of that was spending on online social communication. Pastime activities and instrumental function accounts for 28% and 27%. Calling and SMS only account for 7%. Second，the total time decreased as undergraduates’ grade rises. Third，there was errors between the times they estimate and they actual spent on smart-phone. It means that smart-phone is still an important social communication tool. Social activities through smart-phone is a crucial complement for real life interpersonal interaction. At last，the enlightenment for time management and future research were discussed.