Abstract: Based on the calculation method of blasthole wall pressure in water-coupled blasting and stress wave propagation theory, a computational model for water-coupled presplitting blasting under in-situ stress conditions was established for the layered excavation of high sidewalls in deep underground powerhouses of hydropower projects. The influence of explosive parameters, rock properties, decoupling coefficient, and in-situ stress levels on fracture formation in water-coupled presplitting blasting was analyzed. The research results indicate that the circumferential tensile stress peak induced by presplitting blasthole exhibits a "W"-shaped distribution along the blasthole connection line. Effective fracture propagation can be achieved only when the minimum circumferential tensile stress at the trough of the "W" curve exceeds the sum of the rock tensile strength and the in-situ stress perpendicular to the blasthole axis. Compared with hard rock, the in-situ stress level has a greater influence on the selection of blasthole spacing in water-coupled presplitting blasting for soft rock, while in hard rock, the properties of explosives play a more significant role in determining the optimal blasthole spacing. Based on the analysis of these influencing factors, dimensional analysis was employed to establish calculation formulas for the optimal blasthole spacing in conventional presplitting blasting and water-coupled presplitting blasting under in-situ stress conditions. Engineering case studies have verified the rationality of the proposed formulas and related computational parameters.