Abstract: Helical cruciform fuel is a promising alternative to conventional rod-bundle fuel because its special geometry can enhance coolant mixing, increase heat transfer area, and eliminate spacer grids through self-positioning. However, gas-liquid two-phase flow characteristics in this geometry remain insufficiently understood, particularly flow pattern transition and void fraction distribution. In this study, air-water two-phase flow experiments were conducted in a 2.7-times enlarged helical cruciform fuel assembly model designed according to the Laplace-length similarity criterion. A double-layer wire-mesh sensor with a 26×26 electrode array was used to measure cross-sectional phase distributions at three representative axial positions. In total, 42 operating conditions covering a wide range of superficial gas and liquid velocities were tested. The raw wire-mesh sensor signals were calibrated with single-phase water data, corrected for overshoot effects, and converted into local void fraction using the Maxwell relation. Bubble size distribution and interfacial area concentration were further obtained from the reconstructed three-dimensional void fraction matrix. The reliability of the measurements was verified by comparing the superficial gas velocity calculated from bubble volume data with the flowmeter values, with most deviations within ±15%. The results indicate that four typical flow patterns occur in the helical cruciform fuel assembly, namely bubbly flow, cap flow, cap-churn flow, and churn flow. A flow-pattern map and corresponding transition criterion were established for this geometry. Compared with a conventional rod-bundle channel, the bubbly-to-cap transition boundary shifts markedly toward lower superficial gas velocities, and cap flow more readily develops into cap-churn flow and churn flow as gas velocity increases. This behavior is attributed to the geometry-induced transverse mixing and cross flow of the helical cruciform structure, which promote gas migration and accumulation in the central subchannels. Under low-void-fraction conditions, dispersed bubbles gradually migrate toward the central channel, forming a core-peaked void-fraction distribution after sufficient axial development. As the flow pattern evolves, the peak void fraction increases from less than 0.25 in bubbly flow to about 0.60 in churn flow. During the evolution from bubbly flow to cap-churn flow, Group-1 bubbles contribute increasingly to the interfacial area concentration, indicating that bubble breakup plays an important role. In churn flow, the interfacial area concentration becomes relatively stable along the axial direction, suggesting a dynamic balance between bubble breakup and coalescence under high-void-fraction conditions. These findings provide experimental evidence and theoretical reference for thermal-hydraulic design and safety analysis of helical cruciform fuel assemblies.