The invention relates to an end-to-end available bandwidth estimating method based on a TCP acknowledgement packet pair. After a receiving end receives a data packet sent by a sending end every time, an ACK packet pair is returned to the sending end; the sending end receives the ACK packet pair, the internal arrival time interval of the ACK packet pair is recorded, the end-to-end bandwidth with the formula shown in the specification is calculated, BW is the end-to-end bandwidth, L is the volume of the ACK packet pair, and delta T is the internal arrival time interval of the ACK packet pair; the background flow rate BWback is calculated through an ACK detection interval model; the end-to-end available bandwidth AW conforms to the formula: AW=BW-BWback. The sample selection strategy is improved, the time-delayed sum (TDS) concept is introduced, the sample with the minimum TDS is selected to carry out the available bandwidth estimation to lower the effect of the background flow to the estimation result, and therefore the bandwidth estimation accuracy is guaranteed. The data packet pair technology is ingeniously adopted in the ACK based on the TCP, the ACK packet pair replaces a detection packet pair in the packet pair technology, and therefore the cost for the bandwidth estimation is greatly reduced, and the accuracy of the bandwidth estimation is facilitated.