Blue-phase liquid crystals are self-assembled three-dimensional soft photonic crystals with a wide range of applications in the fields of sensing and display, information encryption, anti-counterfeiting, and optical devices. However, the research on the blue-phase phase transition is not sufficient. In this paper, phase transition process was investigated for the single-domain BPⅡ to BPⅠ (a wide temperature interval 2.5 ℃) based on optical and structural features, and the mechanism of phase transition behaviors at each stage was elucidated by using the crystal nucleation and growth theory. BPⅡ-BPⅠ coexistence stage was proved by the bimodal peaks of the reflectance spectra and the superposition of circles and quadruple arcs in the Kossel diffraction pattern, which indicated simple superposition of the BPⅡ and BPⅠ phase state. The lamellar growth structures of the BP‍Ⅱ, BP‍Ⅱ-BP‍Ⅰ, and BP‍Ⅰ phases were observed using polarized light microscopy and low-voltage scanning electron microscopy. BP‍Ⅰ was found to nucleate and grow epitaxially from the edge dislocations of BP‍Ⅱ, and its phase transition process transformed layer by layer from top to bottom of the cell. The edge dislocations and screw dislocations moved and merged at the BP‍Ⅰ stage, transforming the texture into a concentric ring. This study is of great significance both for enrich the theory of blue phase transition and guiding the preparation of large-area single-domain blue phases.