Scale steel square is simple, basal length measurement equipment. But it is widely used in our lives. In order to ensure the accuracy and consistence of value transmitting, this article introduces the scale steel squares value tracing method, and analyses the verticality and line scale result which contains the analysis of maximum permissible error and uncertainty. The uncertainty is related to the measurement principle, step and method, according to the sequence of mathematics model, variance and transmission coefficient, sensitivity coefficient, measurement uncertainty source analysis, measurement uncertainty calculation, combined and extended measurement uncertainty calculation.