Abstract Highly-efficient solar steam generator holds promise in seawater desalination and wastewater treatment with low energy consumption. The efficiency of generator highly depends on several factors including the kind of the absorber layer, the structure and the thickness of the generators. In this study, a model for calculating the optimal thickness was established based on the water transfer rate and thermal conduction loss. An optimal thickness of generator was determined by the calculation and experiment, taking carbonized wood based solar steam generator (CWSG) as an example. CWSG with optimal thickness about 22 mm performed a highest evaporation rate of 6.89 kg m−2 h−1, corresponding to efficiency of 87.7% under 5 sun. CWSG also served for seawater desalination and dye removal, which exhibited a stable performance even after 20 cycles. The results indicate that maximizing the efficiency of solar steam generator by thermal calculation could provide a new choice to design highly efficient solar steam generator for seawater desalination and dye removal.