Atomic-layer-thick two-dimensional transition metal dichalcogenides (2D TMDs) exhibit unique electronic structures with versatile phase-dependent tunability. The structural transition between trigonal (T) and hexagonal (H) phases critically modulates their properties, yet the underlying microscopic mechanisms demand rigorous elucidation through integrated experimental and theoretical investigations. Herein, we demonstrate that domain coalescence angles during growth govern the phase transition efficiency from monolayer 1T-TaS₂ to 1H-TaS₂; when adjacent domains merge at 60°, atoms at the grain boundary naturally rearrange to form 1H-phase nuclei. Density functional theory (DFT) calculations reveal that charge asymmetry at domain boundaries drives the preferential unidirectional growth of the 1H phase from these nucleation sites. By employing rapid cooling (~550 K·min^-1) to shorten the time window for phase conversion, we successfully suppressed the 1T-to-1H phase transition and synthesized large-area monolayer 1T-TaS₂ (≥180 nm × 100 nm), reducing the post-coalescence H-phase formation ratio from 75% to 24%. This study comprehensively deciphers the microscopic mechanisms of the 1T-to-1H phase transition via coupled experiments and theory, which possesses generalizability to TMDC materials, provides a reliable phase modulation strategy, and expands the methodology for precise microscopic-scale phase engineering.