Abstract: In recent years, low-orbit satellite constellations have been rapidly developed. They will play an increasingly important role in the military and civilian fields. How to improve the bandwidth utilization of low-orbit satellite networks is of great significance for them to play a valuable role. The traditional TCP (Transmission Control Protocol) protocol and its variants are designed for terrestrial networks. They cannot adapt to low-orbit satellite networks with long delay, high bit error rate, and high dynamic changes. Therefore, in order to fully utilize the bandwidth resource of low-orbit satellite networks and thus enable high-speed services to be carried, new transport control protocol needs to be designed according to the characteristics of low-orbit satellite networks. In this paper, we firstly analyze the characteristics of low-orbit satellite networks and the problems of existing transport control protocols in satellite networks. Then, a novel congestion control algorithm, called DDTCP (delay-differentiated TCP), based on path information estimation and delay differentiation is proposed. The path delay in low-orbit satellite networks may be caused by a variety of factors. Next the delay information of the past period of time in DDTCP is stored at the source. Finally, a path delay differentiation mechanism is proposed and the congestion window will be adjusted according to the classification results. In this way, a reasonable congestion window can be set quickly to avoid link cache overflow or throughput degradation after a change in network conditions. The experimental results show that the new transport control protocol achieves higher and more stable throughput in low-orbit satellite networks, and the throughput improvement in DDTCP is more than 19% compared with that in the traditional congestion control algorithms.