Abstract: Helically coiled tube (HCT) is widely used in nuclear engineering and other fields due to its effective compactness, excellent structure robustness, heat transfer efficiency and flow stability. As one of the most important gasliquid twophase flow patterns in HCT, the characteristic of annular flow has a great influence on the heat transfer performance of helical oncethrough steam generator. Especially, the circumferential distribution of liquid film thickness in annular flow directly determines the dryout onset location. Although the liquid film flow in straight tubes has been extensively studied, due to the limitation of measurement technology, the precise measurement data of liquid film thickness in HCT are still very rare. In this paper, based on the selfdeveloped noninvasive contact ringisland array sensor (RIAS), airwater twophase flow experiments were carried out to study the threedimensional spatialtemporal distribution of liquid film thickness in vertical HCT with different structures. According to the results, four typical liquid film flow regimes were defined based on the circumferential position of the thicker liquid film appearing in the tube wall. That is, the bottom distribution (BD) dominated by gravity force, the outside distribution (OD) dominated by liquid centrifugal force, the inside distribution (ID) dominated by gas centrifugal force and the insideoutside distribution (IOD) dominated by secondary flow. The effects of coil diameter and coil pitch of HCT as well as gas and liquid velocities on the distribution of liquid film flow regimes were detailed analyzed. It is implied that the coil diameter determines the intensity of centrifugal force and secondary flow. Generally, the smaller the coil diameter, the easier the circumferential distribution forms of OD, ID and IOD are to appear. Also, increasing the coil pitch weakens the effect of centrifugal force on the liquid film. Then a small coil pitch normally results in the circumferential distribution form of OD while it is ID for large coil pitch. It is also found that the IOD occurs only when both gas and liquid velocities are large enough. By taking the influence of HCT geometry and fluid parameters into account, a general submap of liquid film flow in HCT was proposed using the modified Dean number De* and the modified LockhartMartinelli parameter X*. The transition criteria between different liquid film flow regimes were also established and verified with experimental data in present work and previous references. From this work, the mechanism of curvatureinduced centrifugal force and secondary flow on the spatiotemporal evolution of liquid film in HCT is revealed.