NOVELTY - A biomass is provided into furnace end pre-heating segment of pyrolysis and gasification converter, preheated and subjected to pyrolysis and dry distillation, to obtain dry distillation gas. Then, charcoal is reacted with water gas and semi-water gas which are provided into rotary internal barrel to mix with distillation gas, to obtain pyrolysis gas which enters water sealing dust remover, cooled and exhausted. Thus, preparation of coal gas by internal heating method is carried out.    USE - Preparation of coal gas.    ADVANTAGE - The method provides coal gas having improved thermal property.    DETAILED DESCRIPTION - A biomass is provided into furnace end pre-heating segment of pyrolysis and gasification converter from a feed port, moved in the furnace from right to left, subjected to heat exchange with rough pyrolysis gas from pyrolysis segment of converter material from left to right, dried and preheated while temperature of rough pyrolysis gas after heat exchange is lowered to 70 degrees C, and then rough pyrolysis gas is discharged from rough pyrolysis gas outlet to purify, where one portion of purified gas is used as return gas which is returned to combustion device for burning with air and generated heat energy is provided through flue to heat biomass in rotary internal barrel through rotary internal barrel wall, and other portion of gas enters gas cabinet. Then, pre-heated and dried biomass is provided in pyrolysis segment of converter material in rotary internal barrel of pyrolysis segment of converter material, and biomass undergoes pyrolysis at 600-800 degrees C and dry distillation by heat energy generated by mixed combustion of the return gas and air in the external flue to form charcoal, to obtain dry distillation gas. Then, charcoal which has undergone pyrolysis and dry distillation is provided to cooling segment at the end of converter to react with cooling water, water gas and semi-water gas having thermal values of more than 10000 KJ/NM3 which are generated in the process of absorbing heat energy, then, water gas and semi-water gas are provided into rotary internal barrel to mix with dry distillation gas, to obtain rough pyrolysis gas which is discharged through rough pyrolysis gas outlet after exchanging heat with furnace end pre-heating segment, charcoal is lowered to 200 degrees C from 600-800 degrees C, and discharged from charcoal discharge port. Then, rough pyrolysis gas from rough pyrolysis gas outlet enters water sealing dust remover for removing dust, separated gas is purified by gas collecting tank, cooled by spray cooling tower and then exhausted by an exhaust fan, where one portion of the gas returns to converter as return gas, enters combustion device from return gas inlet and burns with air, and other portion of gas enters gas cabinet. The oil-containing washing water from the water sealing dust remover and spray cooling tower enters tar separator for separating tar, while separated water is circulated. Thus, preparation of coal gas by internal heating method is carried out. An INDEPENDENT CLAIM is included for rotary furnace.