The invention discloses a sonar detection method for a water drainage pipeline. The sonar detection method comprises the following steps: S1, getting ready for detection; S2, equipping detection equipment; S3, carrying out sonar detection; and S4, processing data. The sonar detection method has the advantage that detection can be carried out in the water drainage pipeline with certain depth of water, so that the problems that a conventional detection method is unsafe for a detector and is inaccurate to judge functional defects are avoided; moreover, original screening fixed precision is improved to 800 points per circle from 400 points per circle; and an angle that a step motor rotates every time is reduced, so that emitting and receiving times are increased, and therefore, quantity of sampling points is increased, and precision is improved.