A preparation method of the damage of the protection slopes of underwater concrete slabs is applicable to the preparation of the damage of the protection slopes of the underwater concrete slabs of waterways, dikes, dams, bank slopes, etc. In the method, underwater detection is carried out, mould bags with a suitable size are customized according to the position and the shape of a damage hole, mould bag concrete is positioned and casted, after the mould bag concrete blocks the damage hole, an infusion hole and a water outlet are bored at proper positions above the damage hole, a duct is applied to infuse underwater non-dispersible concrete from the infusion hole, the self-compaction and anti-dispersibility of the underwater non-dispersible concrete is utilized to fill up the cavity, seeper is drained out of the water outlet, consequently, the cavity is filled up, and the rupture and the collapse of the damaged slab can be prevented. The present invention does not need to build a cofferdam, underwater human operation is relatively less, engineering quantities and construction difficulties are reduced and the cost is decreased; because the underwater non-dispersible concrete is utilized in the concreting construction, the construction is safe and the water body is not polluted; a waterway is not needed to be closed up, so navigation is not affected.