A hot video prediction system is provided. A video comments database stores video comments submitted by a plurality of users. A user social network constructor establishes a user social network according to the video comments. When new comments of a new video are received, a hot video predictor uses the user social network to determine a similar theme between the new video and hot videos that have been hot for a period of time, and predicts whether the new video will become popular accordingly. A social network adaptor checks the prediction, and modifies the user social network accordingly.