A detection ruler for a building site relates to a detection device, and is formed by a ruler body, the tail end of the ruler body is hinged at one hinge point of four head-and-tail hinged frames with the same specification, the ruler body is in a movable fit with and sleeved with a sleeve, the sleeve is fixedly connected to the hinge point of the two frames corresponding to the tail end of the hinged ruler body, the front end of the ruler body is provided with a telescopic tube, and the sleeve is provided with a locking device. The detection ruler is advantaged by simple structure, low manufacture cost, easy operation, obvious and visual detection structure, labour saving and time saving, improvement of work efficiency, small size, portability, and easy carrying.