BitTorrent employs a static quota-based peer selection strategy to fixedly allocate upload quotas for the Tit-For-Tat (TFT) choke algorithm and Optimistic Unchoke (OU) algorithm, which would incur a paradox of supply and demand between upload peers and request peers, where many request peers stay in the starvation state while many upload quotas stay in the idleness state. We propose a dynamic quota-based peer selection strategy, where request peers are classified by the principle of investment return and according to dynamic changed numbers of different request peers an upload peer adaptively allocates upload quotas for the TFT and OU algorithms. Simulation experiment shows that our dynamic quota allocation scheme can eliminate the paradox of supply and demand, increase the resource utility ratio of upload peers, and decrease the file download time, at the cost of uploading more a few file blocks for each upload peer.