NOVELTY - Coal is supplied into furnace, and ground carbonate supplement is supplied into furnace, to obtain mixed charge. The mixed charge is added into carbonization furnace, and subjected to medium-temperature carbonization and high-temperature carbonization. The resulted product is heated, thermally decomposed, heated, quenched, and screened, to obtain solid clean fuel. The carbonization furnace carbonization gas overflow is heated, tar is removed, subjected to wet flue gas desulfurization process, washed, carbon is pretreated, desulfurized, and purified, to obtain methane synthesis gas.    USE - Preparation of methane synthesis gas (claimed).    ADVANTAGE - The methane synthesis gas is prepared by simple and economical method.    DETAILED DESCRIPTION - Quality indicators of furnace are determined, where FCd is 50% or more, Vadf is 25% or more, Ad is 25% or less and St,d is less than 1.5%. The burning rate of carbon material of carbonate supplement is determined. Coal is supplied into furnace, and gas production rate and original carbonization gas are determined, Then, carbonate supplement is subjected to dry grinding into particle size of less than 80 mesh, and supplied into furnace, to obtain mixed charge. The mixed charge is added into carbonization furnace, and subjected to medium-temperature carbonization, such that the amount of carbon material of carbonate supplement is 0.5-15 wt.% with respect to the amount of coal in the furnace. The resulted product is subjected to high-temperature carbonization, such that the amount of carbon material of carbonate supplement is 4-20 wt.% with respect to the amount of coal in the furnace. The resulted product is heated, thermally decomposed, heated continuously at carbonization temperature for 6-12 hours, furnace charge center temperature is increased to 700-900 degrees C, subjected to high-temperature carbonization heating for 12-24 hours, furnace charge center temperature is increased to 900-1100 degrees C, quenched to room temperature, and screened, to obtain solid clean fuel. The carbonization furnace carbonization gas overflow is heated, tar is removed, subjected to wet flue gas desulfurization process, washed using ammonia, benzene is recovered, net carbon is pretreated, subjected to desulfurization, and purified, to obtain methane synthesis gas. The carbon-hydrogen ratio (R') of methane synthesis gas is 3-3.3, which is calculated by R'=(H2-CO2)/(CO+CO2), sulfur content is 0.1 ppm or less and aromatic hydrocarbon content is 20 mg/m3 or less.