The invention discloses a cross-platform user identification method and a cross-platform user identification system, which take the importance of use messages in social platforms into full consideration and identify whether a user is the same user according to the similarity of personalized information, such as user knowledge, interests, preferences, writing styles and wording habits, reflected by the user messages in two accounts of different platforms within a corresponding period of time. Specifically, the method comprises the steps that obtains message contents, which are released within a preset period of time, in the two accounts of the different platforms are obtained, word segmentation and feature extraction treatment are carried out on the message contents of the two accounts, and on the basis, by utilizing the similarity between the segmented word features of the messages of the two accounts, whether the two accounts of the different platforms belong to the same user is identified. Thus, the cross-platform user identification method and the cross-platform user identification system solve the problem of how to identify the same user on different social platforms, and further provide support for the analysis of cross-platform data of the same user.