Multi-Path Transmission Control Protocol (MPTCP) is emerging as a dominant paradigm that enables users to utilize multiple Network Interface Controllers (NICs) simultaneously. Due to the complexity of its protocol design, the steady-state performance of MPTCP still remains largely unclear through model analysis. This introduces severe challenges to quantitatively study the efficiency, fairness and stability of existing MPTCP implementations. In this paper, we for the first time investigate the modeling of coupled congestion control and flow control algorithms in MPTCP. By proposing a closed-form throughput model, we reveal the relationship between MPTCP throughput and subflow characters, such as Round Trip Time (RTT), packet loss rate and receive buffer size. The extensive NS2-based evaluation indicates that the proposed model can be applied to understand the throughput of MPTCP in various situations. In particular, when MPTCP subflows have similar RTTs, the average Error Rate (ER) of the proposed model is less than 8%. Even in the situation where huge RTT difference exists between subflows, the model can still behave well with average ER less than 10%.