The invention discloses a cofferdam sludge suction sinking method. An air sludge suction machine is adopted to suck sludge, and in the sludge sucking process, water is jet to the position of a sludge suction opening in a high-pressure manner; and in the sludge sucking process, a cofferdam in a foundation trench gradually sinks; in the sinking process of the cofferdam, the sludge suction position of the air sludge suction machine in the foundation trench is adjusted according to cofferdam displacement and inclination situations to guarantee that the cofferdam stably sinks within an allowed range; in the sludge sucking process, the external and internal water level difference of the cofferdam is observed at any time, water is supplemented at any time to guarantee that the water level difference is small enough, and large sand turning is avoided; with the increase of the depth of inlet water of the cofferdam, the pull cable stress state of the cofferdam is appropriately adjusted so that the cofferdam can keep vertical all the time; in the sinking process of the cofferdam, a diver is arranged to monitor the soil texture situation of cofferdam blade feet, if a boulder or rock stratum geology exists, local underwater loosening blasting is carried out; and after the cofferdam sinks in place, pull cables on the cofferdam are removed. Compared with the prior art, the method has the beneficial effects that the sinking construction progress of the cofferdam can be accelerated, and the construction cost is saved as a whole.