The invention discloses a silt underwater repose angle measurement method. The silt underwater repose angle measurement method comprises the steps of adding silt on a circular truncated cone in a detection container through a funnel and forming a sand pile underwater, wherein the radius of the circular truncated cone is measured in advance. The silt underwater repose angle measurement method is characterized in that according to the theory of communicating vessels, height measurement is converted into water level measurement, that is, the liquid level is controlled to gradually drop to be flush with the top of the sand pile by controlling a water injection container communicated with the bottom of the detection container, the height of the sand pile is obtained by detecting the height of the water level from above the water surface, and then calculation is conducted to obtain a silt underwater repose angle. The method has the advantages that the adopted devices are simple in structure, the measurement accuracy is high, the fault tolerance is good, operation is easy and convenient, and control is convenient.