A method for measuring an asymmetric delay of a transmission link, comprising: performing, by a slave terminal, clock synchronization with a master terminal, wherein each of the master terminal and the slave terminal is provided with a switch module for changing optical fiber connection on a link through a respective switchover module inside each of the master terminal and the slave terminal without performing a manual switchover comprising plugging in or pulling out any optical fiber; receiving, by the slave terminal, a first message from the master terminal, and recording a second timestamp of receiving the first message, wherein the first message includes a first timestamp of sending the first message; performing switchover, by the slave terminal, with the master terminal by exchanging messages with the master terminal to change optical fiber connection through the switch module; when the switchover is completed, receiving, by the slave terminal, a second message from the master terminal, and recording a fourth timestamp of receiving the second message, wherein the second message includes a third timestamp of sending the second message; and calculating, by the slave terminal, an asymmetric delay T of a transmission link according to the first timestamp, the second timestamp, the third timestamp, and the fourth timestamp, wherein T=(t1-t2)-(t3-t4), t1, t2, t3 and t4 being the first timestamp, the second timestamp, the third timestamp, and the fourth timestamp, respectively; wherein the step of performing switchover with the master terminal by exchanging messages with the master terminal to change optical fiber connection comprises: after the slave terminal receives the first message, sending, by the slave terminal, a first switching message to the master terminal, so that the master terminal sends a second switching message to the slave terminal according to the first switching message, and performs switchover to change optical fiber connection after the second switching message is sent; receiving, by the slave terminal, the second switching message from the master terminal; and performing switchover, by the slave terminal, to change optical fiber connection according to the second switching message. performing, by a slave terminal, clock synchronization with a master terminal, wherein each of the master terminal and the slave terminal is provided with a switch module for changing optical fiber connection on a link through a respective switchover module inside each of the master terminal and the slave terminal without performing a manual switchover comprising plugging in or pulling out any optical fiber; receiving, by the slave terminal, a first message from the master terminal, and recording a second timestamp of receiving the first message, wherein the first message includes a first timestamp of sending the first message; performing switchover, by the slave terminal, with the master terminal by exchanging messages with the master terminal to change optical fiber connection through the switch module; when the switchover is completed, receiving, by the slave terminal, a second message from the master terminal, and recording a fourth timestamp of receiving the second message, wherein the second message includes a third timestamp of sending the second message; and calculating, by the slave terminal, an asymmetric delay T of a transmission link according to the first timestamp, the second timestamp, the third timestamp, and the fourth timestamp, wherein T=(t1-t2)-(t3-t4), t1, t2, t3 and t4 being the first timestamp, the second timestamp, the third timestamp, and the fourth timestamp, respectively; wherein the step of performing switchover with the master terminal by exchanging messages with the master terminal to change optical fiber connection comprises: after the slave terminal receives the first message, sending, by the slave terminal, a first switching message to the master terminal, so that the master terminal sends a second switching message to the slave terminal according to the first switching message, and performs switchover to change optical fiber connection after the second switching message is sent; receiving, by the slave terminal, the second switching message from the master terminal; and performing switchover, by the slave terminal, to change optical fiber connection according to the second switching message. after the slave terminal receives the first message, sending, by the slave terminal, a first switching message to the master terminal, so that the master terminal sends a second switching message to the slave terminal according to the first switching message, and performs switchover to change optical fiber connection after the second switching message is sent; receiving, by the slave terminal, the second switching message from the master terminal; and performing switchover, by the slave terminal, to change optical fiber connection according to the second switching message.