Abstract: As a main indicator of structural stability, the total potential energy equation for thin-walled members is widely referred to in academeic research and practical design of steel structures. The total potential energy equation has many forms adopted in different codes. In the context of rational mechanics, this paper adopts the expression for longitudinal displacement, thus to compare several typical forms of the total potential energy equation. It is found that the difference in forms is mainly due to fundamental assumptions, the calculation of longitudinal displacement, the derivation method, and the imperfection of engineering beam theory. Among them, the classical form of the total potential energy equation for thin-walled members is the most accurate and rational.