Acid treatments aimed at reducing formation breakdown pressure are becoming increasingly popular in stimulating deep and ultra-deep geothermal and natural gas reservoirs, where high pumping pressures are typically required for fracturing. This technique effectively reduces surface pumping pressures, ensuring safe operations. Through multiscale experiments and mechanistic analysis, this study reveals the fundamental mechanisms underlying breakdown pressure reduction via acid preconditioning. Key findings include: (1) High breakdown pressures arise from high in-situ stress, low porosity and permeability, and engineering contamination. (2) Acid-induced mineral dissolution triggers dual effects-pore structure evolution enhances reservoir permeability (facilitating subsequent fracturing fluid imbibition and pressure transmission), while mechanical property degradation substantially weakens rock resistance to fracturing. (3) Comparative analysis of HCl, organic acids, and chelating agents demonstrates that high-temperature reservoirs benefit from low-corrosivity chelating agents (e.g., GLDA) or organic acid systems combined with low-concentration HCl, achieving optimal dissolution efficiency while ensuring wellbore integrity. (4) A multiscale laboratory evaluation framework was established to integrate experimental data for optimizing acid formulations and post-acid fracturing strategies. This paper provides mechanistic insights, acid system selection criteria, and experimental methodologies for breakdown pressure reduction in deep carbonate reservoirs, offering significant engineering value for achieving safe and efficient reservoir stimulation.