Abstract: The Online Q&A platform provides a good channel for the public to acquire knowledge and exchange information. This paper aims to construct a method to measure users' demands for information in specific fields based on the Q&A platform, so as to grasp the public's understanding of relevant issues and find out the obstacles and deficiencies of knowledge acquisition. Based on Baidu Knows and taking the field of counter-terrorism as an example, the research outlined the topic, heat and evolution characteristics of the counter-terrorism information needs. The objective and subjective correlations between answers and questions were integrated to evaluate the satisfaction degree of needs. The experimental results were finally analyzed combining the degree of information needs and their satisfaction. It is found that the themes of the public's counter-terrorism information needs are diversified and the fact-based information needs are the best satisfied. The findings can help evaluate the supply-need relationship and actual effect of counter-terrorism information service and knowledge popularization. The analysis framework can effectively support the mining for users’ information needs on Q&A platform, and provide data and decision support for high-quality information services. 

Key words: Information need, Satisfaction of needs, Q&A platform, Counter-terrorism information, User behavior, Public security information