This paper reported an empirical analysis on user behavior dynamics in online forum. The analysis results on the posts show that both the distribution of the reply number of posts and the distribution of different user number of posts follow power-law distributions with heavy tails, while the distribution of the browse number of posts has no laws. We observed positive correlation between browse number of posts and reply number of posts and the ratio of them is bigger than 10. The statistic results of the users' actions show that the post number, the reply number and the number of root posts which a user has posted replies all follow power-law distributions, which means that the user behaviors in online forums are heterogeneous, and most users post or reply rarely while few users post or reply frequently, and that most users have a small range of concerns while few users have a large range of concerns. We also observed that both the distribution of one-user one-day reply number and the distribution of one-user one-post reply number follow power-law distributions, which means that some people submit a large number of replies on a few of days or submit a large number of replies on a few of posts. The findings of this paper may not only provide guides to online user behavior modeling but may also be applied to online public opinion monitoring and online water-army finding.