The invention discloses a sediment underwater repose angle measuring instrument and an application method thereof. The sediment underwater repose angle measuring instrument comprises a transparent main box, a sand adding trough, scale lines and a bearing plate, wherein the transparent main box is shaped as an uncovered cuboid, the sand adding trough is integrally molded with the transparent main box and located at the side surface where the short side of the bottom surface of the transparent main box is located, the sand adding trough is a trough with the side surface being an arc surface, thelength of the sand adding trough is matched with the width of the bottom surface of the transparent main box, the bottom of the sand adding trough is provided with a strip seam at the junction with the transparent main box, the scale lines are located on the outer surfaces of two side surfaces, which are adjacent to the side surface where the sand adding trough is located, of the transparent mainbox, the two side surfaces are provided with two parallel vertical scale lines, the horizontal distance B between the two parallel scale lines is a constant value, the scale lines have already been calibrated on the transparent main box when leaving the factory, the bearing plate is fixedly connected at the inner edge of the transparent main box, and the upper surface is attached with a rubber anti-skid pad with the surface being rough. The sediment underwater repose angle measuring instrument is simple in structure, low in manufacturing cost and simple to operate.