Abstract: This study explores how optimized spacer design enhances heat and mass transfer efficiency in microscale flows within membrane distillation seawater desalination modules. Various microscale flow channels exist inside these modules, such as gaps between spacer filaments and spaces between the membrane, influencing performance significantly. A lab-scale direct contact membrane distillation system is built to assess key parameters including spacer structure, feed temperature, salt concentration, and flow condition on permeate flux. Higher feed temperature markedly increase water production. Introducing spacers improve flow distribution near the membrane surface, reducing both concentration and temperature polarization. Optimized spacer structures under high-temperature and high-salinity conditions boost permeate flux by about 30% compared to configurations without spacers. Using spacer 2, the maximum water production can reach up to 15.38 L/(m²·h) under the following conditions: feed temperature of 60 ℃, saline concentration of 0%, and counter-current flow configuration. Counter-current flow outperforms co-current flow, with further efficiency gains when combined with optimized spacers. At elevated temperatures, bubble formation on spacer surfaces become more pronounced. Microscale roughness provides numerous nucleation sites for vapor, enhancing interfacial phase-change mass transfer. Spacers enhance membrane distillation performance through regulating flow fields, intensifying mass transfer, and providing mechanical support to membranes. These findings offer valuable experimental and theoretical insights for designing efficient membrane distillation systems for seawater desalination.