TCP performance in the IEEE 802.1-based multihop ad hoc networks is extremely poor, because the congestion conrtol mechanism of TCP cannot effectively deal with te problem of packet drops caused by mobility and shared channel contention among wireless nodes. In this paper, we present a cross-layer method, which adaptively adjusts the TCP maximum window size according to the number of RTS (Request To Send) retry counts of the MAC layer at the TCP sender, to control the number of TCP packets in the network and thus decrease the channel contention. Our simulation results show that this mehod can remarkably improve TCP throughput and its stability.