The online social networks have experienced an unprecedented proliferation. Various platforms change the way people learning about information, particularly on ongoing events, which can only be known from mainstream media in the past. The social media platforms have many pleasing properties compared with traditional media: convenient, detailed, fast and interactive. This provides us an opportunity to learn immediate and detailed information about event of interest, which is highly preferred by decision makers and the public especially when emergencies and significant events happen. Keyword search function is provided by social media platforms like Twitter for users to search messages containing the query keyword(s). However, the returned results are piecemeal due to length limitation, could be mixed with irrelevant tweets or are incomplete due to inappropriate query. This calls for research on collecting clean and complete event-related messages from social media platform. In this dissertation, the researches are conducted on Twitter platform. The collected event relevant tweets could be a large First of all, I would like to express my sincere gratitude towards my supervisor Prof. Aixin Sun. Without his patient guidance, encouragement, immense knowledge and advice, I would not be able to finish my Ph.D career. Under his elaborative supervision, I have not only learned the state-of-the-art information retrieval knowledge, but also acquired a conscientious research attitude through critical thinking. The knowledge and methodology I learnt are invaluable to my research, and even to my life. I feel very grateful to Prof. Gao Cong for his introducing me to my supervisor. Without him, I may miss the chance to study in NTU and do research with Prof. Sun. I would also like to express my great thankfulness to SAP for providing me Ph.D scholarship, and the chance to get access to real industry working environment and work with talented colleagues. I appreciate all the supports, guidances and suggestions given