A transmission control protocol (TCP) flow control method is provided, which comprises: sending a data packet from a packet processor to a receiver and storing a copy of the data packet; receiving a current ACK packet with a current packet number; determining whether the current packet number is identical to a last packet number and whether a last substitute ACK packet generated by the input ACK filter exists; and performing steps respectively corresponding to different results of this determination to avoid TCP congestion control timely. A TCP flow control device performing the method is also disclosed.