Quantitative understanding of people's behaviors on social networks has significant meaning to reveal the origins of many socioeconomic phenomena. This paper focuses on the time characteristic of human behavior online. Four famous web forums of China were analyzed, including Sina, NetEase, HuBeiDongHu and LiXiang. The empirical analysis result presents some statistical features on the behaviors of forum users. We found that the intervals of user's post and reply behaviors both follow heavy-tailed distribution and have high burstiness and low memory. But the day activities do not follow heavy-tailed distribution and the values concentrate between 10 and 100. Further, we studied the intra-day post and reply behaviors and showed that people's behaviors online have similar time characteristic with that of real life, and they submit obviously more posts and replies on work time and seldom do actions after midnight. The findings of this paper reveal some interesting features of human behaviors on online forums and are helpful to the behavior-based detection of spam and spammer.