Wireless mesh network (WMN) has been proposed to extend wireless coverage by using multi-hop communication to enable network service for mobile devices via densely deployed mesh APs. While modern wireless devices are capable of using multiple WiFi interfaces, the Multipath TCP (MPTCP) protocol has been employed to make full use of the capacity of multiple radios. A key question of MPTCP communication in WMN is to decide the best access point (AP) association for each wireless interface to achieve maximum network performance. In this paper, we propose an optimal AP association scheme based on goodput measurement. We setup a wireless mesh testbed with densely deployed APs to study the impact of signal strength and multi-hop communication. We fit the measured data by a three-dimensional surface, which shows that network goodput can be estimated by a function of Received Signal Strength Indicator (RSSI) and hops. With the proposed estimation function, we formulate the AP association problem as a mathematical optimization problem and solve it by Integer Programming.We implement the proposed strategy in the real deployed testbed and evaluate its performance using MPTCP, which shows that it improves MPTCP goodput significantly compared to several existing strategies.