Optical packet switching network is expected to be the most hopeful candidate for future high-speed internetworking since it is characteristic of large capacity and fine granularity. However, due to lack of effective memory in optical domain for buffering, packet loss event happens even when the load of network is relatively low. This phenomenon overthrows the prerequisite assumption of TCP, which regards packet loss event as the sign of network congestion and decreases the sending rate accordingly. As a result, unnecessary slow down will adversely affect the throughput of optical packet switching network if TCP is adopted. In this paper, we propose a novel flow control algorithm which adaptively controls packet size together with window size in order to improve throughput for optical packet switching network. We evaluate it by simulation on throughput improvement and other properties including convergence and fairness