The invention discloses a bridge open caisson underwater three-dimensional mechanical scanning sonar monitoring method, and belongs to the field of monitoring methods in sinking construction of bridgeopen caisson foundation. The method comprises the following steps that an unconstructed open caisson compartment is selected, and a three-dimensional mechanical scanning sonar is put down to underwater for scanning; the data acquired by the three-dimensional mechanical scanning sonar are subjected to jigsaw puzzle and denoising through processing software, so that a three-dimensional point cloudthree-dimensional image is obtained; and the needed information is obtained in the three-dimensional point cloud three-dimensional image, wherein the control of the underwater condition of the open caisson is of vital importance. The bridge open caisson underwater three-dimensional mechanical scanning sonar monitoring method aims at overcoming at the defect of a rope measuring method, and the method is objective and reasonable, is scientific and reliable and can be applied in the non-drainage sinking construction of the large open caisson foundation.