An underwater groove pre-splitting blasting construction method comprises the following steps that firstly, early-stage preparation for construction is carried out, and distribution holes are arranged; secondly, measuring and positioning are carried out; thirdly, an underwater environment is simulated onshore to carry out pre-splitting trial blasting; fourthly, vertical holes in step changes are drilled in sequence; fifthly, initiating explosive devices are detected and processed; sixthly, an explosive is charged with the coupling and uncoupling blended explosive charging technology through the same blasting hole; seventhly, a composite blasting network is delayed by milliseconds according to designed connection; eighthly, initiating and blind blasting checking are carried out. By adopting the underwater groove pre-splitting blasting construction method to carry out underwater large-diameter blasting hole pre-splitting blasting in underwater groove construction, the coupling and uncoupling blended explosive charging technology through the same blasting hole is applied, the groove formation is good, slopes are complete, and the harmful blasting effects are controllable.