The invention discloses a device for detecting the slope ratio and a detection method thereof. The device comprises an attaching rod, a measuring rod and a correcting rod, the attaching rod is rotationally connected with the correcting rod, one end of the measuring rod is rotationally connected with the correcting rod, an angle scale and a pointing plate are installed at the end, away from the attaching rod, of the correcting rod, the pointing plate is fixedly connected with the correcting rod, and the angle scale is rotationally connected with the correction rod; one end of the measuring rod is located between the angle scale and the attaching rod, the measuring rod is a multi-section telescopic rod, and a measuring range is arranged on the surface of the measuring rod; a groove is formed in the attaching rod, a sliding block is arranged in the groove, the end, away from the correcting rod, of the measuring rod is detachably connected with the sliding block, a flexible part is connected between the measuring rod and the correcting rod, and the length of the flexible part is suitable for the measuring rod and the correcting rod to form a 90-degree angle; and the device solves the problems that in the prior art, the slope ratio cannot be visually read, the slope ratio can only be monitored at a single point, and the slope ratio cannot be rapidly and effectively measured.