The invention relates to a method for utilizing cross-correlation function to passively measure the connection round-trip delay of transmission control protocol (TCP). A measuring node and a measuringdevice are selected or set on the paths connected by TCP; firstly, bi-directional subgroups which go through the node and are connected by TCP in set time are collected by the measuring device and are stored; next, the set time is evenly divided into a plurality of time lengths of small regions; the subgroup number of two directions in the time length of each small region is respectively counted;the subgroup number of the same direction in the time lengths of all small regions, which go through the node, forms a statistical number sequence; the bi-directional subgroups connected by TCP formtwo statistical number sequences in the same time quantum; thirdly, the value of the cross-correlation function of the two statistical number sequences under different time offset is calculated; the corresponding relationship between each ACK subgroup and data subgroup thereof is determined according to the time offset corresponding to the maximum function value; and finally, the TCP round-trip delay connected in the set time is calculated. The method overcomes the defects in the prior art that the additional throughput needs to be added to network when measuring, and the efficiency is low.