Abstract: To study the effective information of social network,based on the theory of node degree distribution and node strength,a method to analyze the effective information in social network was proposed.First,based on Dunbar's number,the node effective information function(NEIF) together with node strength to describe information patterns of social network was proposed.By combining the NEIF with the node degree distribution function(NDDF) which is obtained from Twitter's social graphic,the social networks' effective information was analyzed.Finally,by using effective information function,the limitations and characteristics of effective information in social networks were estimated,and guidelines for better social networking were provided.The research shows that the Dunbar's number is between 210 and 220,which has exceeded the 150 law.At the same time,the effective information of social network tends to be a specific value.