The utility model discloses a detection ruler for detecting perpendicularity of a building template. The detection ruler comprises a chuck used for being inserted into the outer edge of the building template and clamped, an upper supporting rod, a lower supporting rod and a vertical plate, wherein one end of the upper supporting rod is vertically fixed with the upper end of the chuck; the other end of the upper supporting rod is vertically fixed with the upper end of the vertical plate; a suspension wire is arranged at the upper end of the upper supporting rod; the lower end of the suspension wire is connected with a plumb bob; a rail is arranged on the vertical plate; one end of the lower supporting rod is vertically arranged in the rail; a scale is hinged to the lower supporting rod; and the zero graduation line of the scale is vertically coincided with the suspension wire. The detection ruler disclosed by the utility model is simple in structure, convenient to operate, accurate in measurement and high in measurement efficiency. According to the detection ruler disclosed by the utility model, the detection operation can be finished by one person only, the artificial capital is reduced, the position of the upper supporting rod does not need to be adjusted due to the use of the detection ruler disclosed by the utility model, the perpendicularities at multiple parts can be measured, the operation is convenient and rapid, the labor intensity is reduced, and the safety coefficient is improved.