The invention discloses a method for preparing porous charcoal pieces capable of realizing efficient photo-thermal steam converting. The method comprises the following steps: 1) drying wood block; placing the wood block in a quartz tubular furnace, under nitrogen atmosphere protection, performing high temperature pyrolysis at the temperature of 300-900 DEG C and carbonization to obtain the whole block of carbonized charcoal; 2) cutting the carbonized charcoal obtained in the step 1) along a vertical charcoal tunnel direction or polishing the carbonized charcoal to obtain the charcoal pieces with thickness being 1-3 mm; and 3) using deionized water to performing ultrasonic cleaning on the charcoal pieces obtained in the step 2), and drying the material to obtain the porous charcoal pieces. The obtained charcoal pieces have regular tunnels, are beneficial for steam flowing, have good adhesiveness with water, and can guarantee for transferring the heat to water. The charcoal pieces have light absorptivity being more than 95% in a 250-2500 nm scope, and under light intensity of 1 kW.m<-2>, the photo-thermal steam efficiency can reach as high 91.2%. The porous charcoal pieces have the advantages of low cost, simple operation, and environmental protection, and can be used for sewage processing and seawater desalination.