The invention discloses a construction method for a dam spillway reinforcing structure. The construction method is carried out according to the following steps that 1, in the water flow direction of adam spillway, cylindrical sleeve plate hole positions are formed in the side slopes at the two sides of the dam spillway 1 according to the set distance, and position labeling is carried out; 2, a static force pressing-in method is adopted to correspondingly press the sleeve plates into the cylindrical sleeve plate hole positions formed in the side slopes of the two sides of the dam spillway; 3,grouting pipes are correspondingly pressed into sleeve pipes of the sleeve plates by using the static force in sequence; 4, laying grouting plates on the side slopes on the two sides of the dam spillway, slurry passing holes in the grouting plates are made to be in butt joint with the grouting pipes at the corresponding positions in a one-to-one correspondence mode, and the grouting plate is filled with soil and compacted; and 5, grouting is carried out on the side slopes at the two sides of the dam spillway at the same time, slurry is injected through grouting holes in the upper surfaces of the grouting plates, then the grouting plates filled with the slurry are formed, and meanwhile, the slurry is diffused into the corresponding grouting pipes through the slurry passing holes in the lower surfaces of the grouting plates. according to the V-shaped integrated waterproof reinforcing structure, side slope strength and waterproof performance of the dam spillway are greatly improved.