With the increase of transmission rate and transmission distance, computer and communication industries are gradually transferring to high-speed serial bus, realizing high-speed interconnection between chip chip, board card and backplane. The speed of these high-speed serial buses is increasing from hundreds of Mbps of USB2.0, LVDS and firewire1394 to the current PCI Express G1 / G2 and SATA The number of Gbps of G1 / G2, XAUI / 2xaui and XFI is even up to 10Gbps, which means that PCB manufacturers in computer and communication industries will have higher and higher requirements for impedance control of differential wiring, so PCB manufacturers and high-speed PCB designers are facing unprecedented challenges. This paper will discuss the principle and characteristics of real differential TDR test method based on IPC-TM-650 manual.