Embodiments of the present invention disclose a method and an apparatus for sending a Transmission Control Protocol TCP data packet and a system. The sending method includes: obtaining a first round-trip time of sending a TCP data packet in a network, and determining a second round-trip time; if the first round-trip time is longer than the second round-trip time, using a congestion window determined according to a first algorithm as a first congestion window; or if the first round-trip time is shorter than or equal to the second round-trip time, using a congestion window determined according to a second algorithm as the first congestion window; and sending the TCP data packet by using the first congestion window. In the technical solution disclosed in the present invention, a current congestion window that exists when a first round-trip time is obtained grows to a first congestion window at one go, so that a requirement of a service for a throughput can be better met, and a network bandwidth can be utilized more effectively.