With the wide application of social networks the interactions of users in social networks become more and more convenient, frequent and cheap for the people in real society. The related research of Behavioral science represents that individual emotions can profoundly affect individual behavior, the emotions of others,the group behavior,and decisionmaking of group.The public mood analysis for social networks have get attention by the early researchers about the social networks analysis,and further more they try to predict the financial markets from social networks to obtain the valuable information from the scial networks.This is a very important academic perspective that critical social intelligence information can be obtained through the analysis of social data from social networks and other social data sources.The public opinion analysis for social networks is an important research field. Nevertheless, the corresponding quantitative evaluation indexes of public opinion analysis trough social networks have not yet been developed.The corresponding social computing method is the base for the data analysis.This paper proposes both the concept of a social negative mood index for social networks and its calculation method, making it a fundamental work in the field of public opinion analysis.