The influence of different dominant phases on the gas-sensing properties of metal oxide n-n heterojunctions remains unclear. In this work, two kinds of phase-inverted ZnO/SnO₂ heterojunction composites (SZ and ZS series) are fabricated via a facile solid-state grinding method. Characterization results show that ZS-7 exhibits finer and more uniform grains with a specific surface area of 70.30 m²·g⁻¹. SZ-7 has a high oxygen vacancy content of 26.28%, and its Sn 3d characteristic peaks shift entirely toward higher binding energies, which verifies electron redistribution at the n-n heterojunction interface. Under UV irradiation, room-temperature NO₂ gas-sensing tests reveal that the ZS series exhibits overall superior performance to the SZ series. ZS-7 delivers a gas response of 46.49, with response/recovery times of 35 s/122 s and an ultralow detection limit of 74 ppb. Mechanism analysis indicates that the ZnO/SnO₂ heterojunction facilitates the separation and migration of photogenerated carriers under UV irradiation. The phase-inverted structure further accelerates photogenerated carrier separation and optimizes charge transport, ultimately improving gas-sensing performance. This study provides a novel design strategy for developing high-performance room-temperature NO₂ sensors.