Abstract: The theory of high speed TCP congestion control algorithms are analysized in this paper. The improvement speed of TCP congestion window dynamically with bandwidth of bottleneck connection is modified, the improvement of congestion window is classified into high speed TCP and traditions TCP mode, and the switch between with them depends on the changing trend of RTT. Simulation shows that high speed TCP can modify the congestion window when the bandwidth is not fully use. And it also can change bandwidth from fully used condition into not fully used.