The invention discloses a method, a system and a device for detecting the disconnection of an exterior line of a telephone of a user. The method comprises the following steps: an auxiliary detection device which is unilaterally conducted during the forward excitation is connected with the telephone of the user in parallel on the exterior line of the telephone which is positioned at the user side;a DC power supply is used at the network side for carrying out the forward excitation and the reverse excitation of the exterior line of the telephone; whether the auxiliary detection device is detected or not on a loop of the exterior line is determined by detecting the current of the loop of the exterior line after the forward excitation and the current of the loop of the exterior line after the reverse excitation; and when the auxiliary detection device is detected, the exterior line of the telephone of the user can be determined to have no disconnection, otherwise, the exterior line of the telephone of the user is determined to have the disconnection. The invention can automatically judge whether the exterior line of the telephone of the user has the disconnection and other accidents or not without the manual intervention, thereby improving the efficiency and the accuracy of the fault judgment of the exterior line.