Solar steam generation has attracted increasing attention due to its applications in water purification (e.g., desalination and wastewater treatment). Many strategies have been developed for achieving efficient photothermal conversion materials based on abundant biomass. However, it is challenging for most of these materials to simultaneously facilitate water transport and manage heat well, which results in unsatisfactory evaporation efficiencies under 1 sun illumination. Here, inspired by natural sugarcane stems' bi-functional structures—vascular bundles for mass transport and parenchyma cells for nutrition storage, we use surface-carbonized de-sugaring stems of sugarcane as efficient solar steam generators. The obtained materials have abundant “closed chambers” for thermal insulation (ca. 0.08 W m⁻¹ K⁻¹ in thermal conductivity) and bundles of vertical channels for water transport. These materials achieve an evaporation conversion efficiency up to 87.4% under 1 sun illumination without the usage of additional thermal insulators. This value surpasses all other biomass-derived materials ever reported. Because such bi-functional structures also widely exist in the stems of other Poaceae plants that are renewable and abundant in nature, this strategy is expected to open a new avenue for the future design and fabrication of diverse, more efficient, and cost-effective photothermal-conversion devices.