The invention discloses a ground flatness detection device. The device comprises a fixed marker post, horizontal rulers, a detection structure, a movable marker post, cleaning structures, a rotating sleeve, a limiting plate, a fixed support, a switch, a cleaning motor and a brush, wherein the fixed marker post and the movable marker post are vertically arranged in parallel, the horizontal rulers are mounted on the top sides of the fixed marker post and the movable marker post, the detection structure is installed between the fixed marker post and the movable marker post, the cleaning structures are installed at the bottom of the fixed marker post and the bottom of the movable marker post respectively, by arranging the cleaning structures, dust and sands at the detection vertex position canbe cleaned up, the cleanliness of the ground is guaranteed, and the detection accuracy can be improved. When detection is carried out, the fixed marker post and the movable marker post are positioned, and through cooperation of the detection mechanism, the flat state of the ground can be determined, the inclination angle of the ground can be accurately determined, at the same time, rotary extension can be achieved, multi-directional detection can be achieved, and the detection effect is better.