Upload utilization is one of the most significant performance metrics in BitTorrent system, whereas there has been no accurate model which is suitable for it yet. For this reason, the delicate relation between some key parameters (maximum concurrent uploads, upload bandwidth, TFT round etc.) in BitTorrent is investigated, furthermore the piece diffusion model and the upload utilization model are proposed. Based on these models, we explore the effect of TFT round for the first time and find that the magnitude of TFT round has a significant influence on the transient stage as well as the end stage. Additionally, we show that when the seed’s upload bandwidth is fully utilized, to achieve optimal performance, each peer’s upload bandwidth should be proportional to its maximum concurrent uploads, and the file size should be large enough compared with the swarm size.