The invention discloses an optical fiber loop detection system, which relates to the technical field of detection and can solve a problem that an optical fiber loop detection result is inaccurate. The optical fiber loop detection system is characterized in that a silicon controlled rectifier valve unit is connected to forward voltage; terminal voltage of the silicon controlled rectifier valve unit is detected; a central controller judges whether the terminal voltage is greater than a preset voltage protection threshold or not; if the terminal voltage is judged to be greater than the preset voltage protection threshold, an overvoltage conduction instruction is generated; a protection recovery unit generates an overvoltage conduction pulse; if the terminal voltage is judged to be not greater than the preset voltage protection threshold, an optical fiber testing instruction is generated; a valve base electronic device generates an optical fiber triggering pulse according to the optical fiber testing instruction; a multi-mode star coupler sends the overvoltage conduction pulse or the optical fiber triggering pulse to the silicon controlled rectifier valve unit; a silicon controlled rectifier voltage detecting board detects loop detection signals at two ends of the silicon controlled rectifier valve unit; the central controller detects the pulse width of the loop detection signals; and the central controller determines whether an optical fiber to be detected is of a normal state or not. The optical fiber loop detection system is mainly applied to the process of detecting an optical fiber loop.