The invention discloses a method for judging disappearance of a secondary direct-current power supply of a far-end transformer substation. A power loss alarm contact of the secondary direct-current power supply of the transformer substation is introduced into I/O on-off input of a communication cell measuring and controlling device. The communication bandwidth of MSTP equipment is divided into the protection bandwidth and measuring and controlling bandwidth which are independent from each other, the protection bandwidth is used for conveying protection information, and the measuring and controlling bandwidth is used for conveying access power loss on-off input information of measuring and controlling equipment. A receiving end transformer substation protection device can detect received offside information in real time. The judgment whether a secondary direct-current power supply of an offside transformer substation disappears can be given out in real time by comparing information of the protection bandwidth with information of the measuring and controlling bandwidth. According to the method, a measuring and controlling loop of an independent power supply and a communication loop of independent bandwidth can be utilized to transmit disappearance information of the secondary direct-current power supply. Disappearance of the secondary direct-current power supply of the far-end transformer substation can be accurately recognized in an online mode by comparing the protection bandwidth with the measuring and controlling bandwidth. Thus, decision basis can be provided for following protection strategies or control strategies.