A DC level detection circuit between high speed signal line connecting ports and a system using the same in optical communications is disclosed. Corresponding ports of a high speed signal line each have an additional resistor, where one additional resistor has a resistance significantly greater than that of the other resistor. The smaller resistor is grounded. The larger resistor is connected to a DC voltage source, a low pass filter, and a signal detection port. Thus, when both ports of the high speed signal line are connected, a status of the electrical level detected at the signal detection port changes. The circuit detects the connection state of the high speed signal line without negative effects on signal transmission and is applicable to various circuits, especially plug-in modules and corresponding slots. Thus, the circuit further enables signal port multifunctionality, increases module installation accuracy, and provides higher compatibility for plug-in modules.