The invention provides a method for monitoring sediment quantity by acoustic waves and aims to solve the problem that silt content of a sediment pit cannot be precisely measured. The method comprises the following steps: an acoustic wave transmitting transducer is buried in concrete at the top of the end surface on the upstream side of the sediment pit; a plurality of acoustic wave receiving transducers are uniformly buried in concrete on the end surface, with different elevations, on the upstream side of the sediment pit; the distances Li between the acoustic wave transmitting transducer and the acoustic wave receiving transducers and the time Ti spent by the excitation signals of the acoustic wave transmitting transducer for reaching the acoustic wave receiving transducers are measured, wherein the unit of the time Ti is (Mu)s and the Ti can be read from the acoustic wave receiving transducers; the velocities Vi of all acoustic wave receiving transducers before and after the sediment pit is filled with water are measured; if the velocities Vi are smaller than 1,800 before and after the sediment pit is filled with water, the acoustic wave receiving transducers are not covered by silt; if the velocities Vi are smaller than 1,800 before the sediment pit is filled with water and the velocities Vi are greater than or equal to 1,800 after the sediment pit is filled with water, the acoustic wave receiving transducers are covered by the slit; the range of the piling height of the slit can be determined through the acoustic wave receiving transducer I which has the highest mounting position and is covered by the silt and the elevation corresponding to the acoustic wave receiving transducer I in the slit.