Objective To explore the mediating effects of college students＇ online social support on the relationship between online self-disclosure and internet altruistic behavior.Methods Using cluster random sampling,the self-disclosure questionnaire for Chinese undergraduates with their internet friend,the online social support scale of adolescence and the internet altruistic behavior scale of undergraduates were applied to 351 college students.Results The score of college students＇ online self-disclosure,online social support and internet altruistic behavior were respectively(2.22±0.45),(2.93±0.59) and (2.04±0.46).The college students＇ online self-disclosure,online social support and internet altruistic behavior were positively correlated with each other (P＜0.01).College students＇ online self-disclosure not only predicted internet altruistic behavior directly,but also predicted internet altruistic behavior indirectly through online social support.The direct effects and indirect effects were 0.200 and 0.265.The ratio of the mediating effect on the total effect was 57％.The fit index of mediation model was x2/df=2.87,RMSEA =0.07,CFI =0.94,TLI =0.93,GFI=0.92,IFI=0.94,NFI=0.91.Conclusion The college students＇ online self-disclosure not only affects the interuet altruistic behavior directly,but also can take the same effect on online social support indirectly.