Abstract: In rock excavation engineering, presplit blasting serves as a core technology to protect the integrity and long-term stability of surrounding rock masses, while the initiation delay between presplit holes and main blast holes is the dominant parameter that directly determines blasting contour quality, stress wave transmission law and retained rock damage degree. To reveal the intrinsic dynamic regulation mechanism of initiation delay on presplit blasting process, this study adopted a coupled method of Digital Laser Dynamic Caustics (DLCS) test system and AUTODYN nonlinear dynamic numerical simulation. A series of dynamic blasting tests were conducted on polymethyl methacrylate (PMMA) specimens with prefabricated primary cracks under multiple typical delay conditions, to systematically analyze presplit crack propagation characteristics, blast stress field evolution, and initiation and propagation response of primary cracks in retained rock masses.Results show that detonating main blast holes during the dynamic propagation of presplit cracks changes the stress state at the crack tip, leading to a fracture mode transition from pure tensile Mode I to mixed Mode I-II tensile-shear fracture. This causes significant deflection of crack paths, reduces the straightness and continuity of presplit fractures, and weakens the stress wave isolation effect. When main blast holes are initiated after presplit cracks have completely arrested, the fully formed continuous presplit cracks can effectively block the transmission of main blast stress waves, with the final crack morphology almost consistent with that of single presplit blasting, realizing optimal fracture formation quality. For surrounding rock stability, presplit hole detonation compresses and closes pre-existing primary cracks through stress wave superposition and detonation gas effects. With the increase of delay time, the attenuation of main blast stress waves is significantly enhanced, which effectively suppresses the initiation and secondary propagation of primary cracks in retained rock, thus improving surrounding rock stability. Numerical results are in good agreement with DLCS test data, verifying the reliability of conclusions. This work clarifies the delay regulation mechanism of presplit blasting, providing theoretical support and practical guidance for precise optimization of blasting parameters in rock excavation engineering.