At present, vector diagram was adopted to help the construction and analysis of the PT secondary loop voltage drop’s mathematical model. To solve the defects in the vector diagram geometric analysis method, in this paper, algebraic analysis method was chose. The method was used in the mathematical model re-construction and systematical analysis of the PT secondary loop voltage drop. In the article, the influence of the PT secondary loop voltage drop on the electric power energy metering was elaborated. And it provided a useful help to understand the voltage drop theory for metering technical staff.