BitTorrent is currently the most prevalent peer-to-peer file sharing system. Many researchers study and modify BitTorrent protocol in order to improve its performance. A fundamental problem is the evaluation of those newly proposed protocols. The current methods of studying peer-to-peer systems, such as analytical modeling, discrete-event simulations and deployment on real networks, often are limited in scalability, reproducibility, and accuracy. Moreover, many of them are difficult to achieve complete and accurate evaluation results under a wide range of conditions. Emulation is an effective tool to tackle these problems and it is suitable to study and evaluate the behaviors of BitTorrent-like file sharing systems. Thus, we propose a lightweight emulator, Virtual BT, which is scalable, flexible, accurate and easy to deploy. It adopts a distributed network architecture whose function modules are loose-coupled and easy to be modified in order to study BitTorrent protocol design. More than 200 virtual nodes can be executed on a contemporary personal computer by using process-level virtualization; and every virtual node exchanges data without causing any disk I/O overhead. Through experiments, Virtual BT demonstrates its effectiveness and gives accurate predictions that closely match the results observed from real network measurements.