The understanding of the liquid-gas flow and heat transfer in the high-aspect-ratio microchannel is very important to realize the high-efficiency phase change chip cooling. In this work, a novel ultra-high-aspect-ratio microchannel with capillary-structured wall was developed to enhance the evaporation heat transfer in microchannel, in which the capillary grooves on the side walls (capillary-structured wall) were designed to avoid the dryout phenomenon. A three-dimensional VOF model was established to predict the immiscible gas-liquid flow in microchannel. The influences of wettability of capillary grooves on the gas-liquid two-phase flow behavior in microchannel were investigated based on the numerical predictions. The slug bubble can be observed for different inlet flow conditions. Variation of pressure loss between inlet and outlet of microchannel with time were studied for different flow rates and gas-liquid ratios. The results show that the existence of capillary structured wall has a significant influence on the liquid-gas two-phase flow behavior in the microchannel. The liquid flow in microgrooves is driven by the capillary force, which can supply more liquid to the side wall to promote the evaporation heat transfer process. The design of capillary-structured wall for ultra-high-aspect-ratio microchannel in this work provides a new approach to improve the performance of the chip cooling technique with microchannels.