The invention discloses a carbon offset method for methane synthesis gas prepared from dry distillation gas. The method is implemented through that a carbonate carbon offset raw material with a mineral powder particle size of less than or equal to 80 meshes is added into retort coal and uniformly mixed, the obtained mixture is added into a dry distillation furnace to heat, red heat furnace charges are cooled to room temperature, the obtained product is sieved so as to obtain a solid clean fuel, and produced CO2 and CO gases are directly fed into the dry distillation gas to match with excess H2 gas, so that the hydrogen-carbon ratio requirement for the synthesis of synthetic oil is met, thereby achieving the purpose of carbon offset. According to the method disclosed by the invention, carbonate minerals are directly added into the retort coal, and a carbon offset effect on dry distillation gas is achieved by using CO2 and CO gases produced by thermal decomposition, therefore, the method has the characteristics of rich raw material sources, simple technological processes, equipment investment saving, low carbon offset cost, good economical efficiency and the like, and can be widely applied to the comprehensive utilization projects of high and medium temperature dry distillation gas.