Recent few years, many p2p VoD softwares are widely applied through out the internet, which gives us an opportunity to study user watching behavior of p2p VoD system at large scale. Many previous works on this area directly regard peer's information transmitting log such as buffer message as peer's watching behavior log. However, the effect of prefetch strategy is not considered. In this paper, we establish a simple mathematical model to show the relationship between the distribution of watching index and bitmap index. Through this model, we attempt to enhance our understanding on p2p VoD user watching behavior and its implications for file sharing efficiency. This paper will be the first study on this problem upto-date. Experiment results confirm the conclusion that the differences between watching index and bitmap index cannot be neglected. A more accurate way to study user watching behaviors can be generated from our model.