Abstract: The addition of micro-groove structures on superhydrophobic surfaces is conducive to the formation of gas-liquid interface under the action of surface tension, which greatly reduces the flow resistance and shows great application potential in flow drag reduction. However, due to the effects of fluid shear and air dissolution, the air in the micro-groove is prone to loss, resulting in a shift in the position of the air-liquid interface toward the bottom of the groove, and thereby reducing the drag reduction effect. To reveal the influence of the position of air-liquid interface on the drag reduction after the air loss in the micro-groove, high-speed microscopic photography experiments and computational fluid dynamics numerical simulations were carried out to study the influence of air-liquid interface positions on the two-phase flow field characteristics, and the variations of the equivalent slip length and the drag reduction rate were also analyzed. The results show that the fluid above the gas-liquid interface changes from attached flow to separation vortex as the gas-liquid interface moves downward, and the direction of the vortex inside the air also changes from clockwise to counterclockwise. The critical air loss rate for the formation of the separation vortex ish^*_\mathrmc = 0.3 ~ 0.4, and the drag reduction rate is reduced from 12% to 5%. The Reynolds numberRe, microchannel widthWand groove length proportionδalso have important effects on the drag reduction performance of microgrooves after the interface descent. The results of this study could help to deepen the understanding of the drag reduction of two-phase flow in micro-grooves on superhydrophobic surfaces, and provide important theoretical guidance for the design and application of micro-groove structures.