The invention provides a method and device for determining power transmission line losses based on a transformer signal. According to the invention, the existing mode of reading the power from the electric energy meter is abandoned. An error analysis is carried out on a voltage transformer participating in line loss statistics, so that the line loss abnormity caused by the own out-of-tolerance value of the voltage transformer is avoided; and on the premise that the transformer accuracy is ensured in detection, all powers of a line are obtained, the obtained powers are compared with powers obtained by processing an opposite station at the same line by using the same method, and thus actual line loss data of the line are obtained by the analysis. According to the method and device provided by the invention, on the premise that whether the error of the voltage transformer exceeds a threshold value is determined, line loss analysis and calculation are carried out by a secondary output signal of the transformer, so that the existing condition of acquiring the line losses from the electric energy meter in the prior art is changed completely and the accuracy of line loss statistics is improved; and preliminary online detection of the error of the voltage transformer in the substation is realized and a phenomenon that existing voltage transformer error detection needs a power-off offline state is changed.