Abstract Solar steam generation has become one of the most promising techniques to realize water purification and desalination owing to the abundance and cleanness of solar energy. Here, we proposed a preheating chamber structure for high-efficiency solar steam generation with carbonized sawdust beds as the solar absorber. Effects of bed porosity on the water transportation, heat transfer and evaporation performance were studied. Experimental results turn out that with the increase of the bed porosity, the heat conductive loss will be increased while water transportation is promoted. When the positive effect of the increased water transportation and the negative effect of the increased heat losses on the evaporation are balanced, the evaporation efficiency reaches the maximum of about 91.5% at the porosity of 76% under one sun illumination in this work. In addition, the influence of water supply on the steam temperature under 3-sun irradiation was also investigated. It turns out that the evaporation efficiency and temperature can be coordinated by adjusting the water-supply area. When the ratio of water-supply area to the evaporation area reaches 58.8%, a proper balance between efficiency (33.8%) and steam temperature (115.8 °C) can be achieved. This work is expected to supply a method to obtain high temperature and also efficiency steam generation.