After a course examination on structural mechanics, a step-by-step score analysis was carried out on a test question aiming at examining the students’ displacement method application. The result of the analysis showed that the main reason for the failure in solving the problem was that most of them can not obtain the stiffness coefficients and free terms of the essential equation based on the displacement method correctly. This paper presents a symbolic rule to eliminate the drawbacks of mistaking the stiffness coefficients and free terms of basic equations that have 2 or more basic unknown quantities. The rule requires that the reference direction for the displacements and constraint forces taken placed at the additional constraints unchanged, meanwhile all constraint forces generated at the same additional constraint should have the same reference direction.