The invention discloses a method for manufacturing gas through biomass and urban waste. The method comprises the following steps: respectively communicating upper sections of hearths of cracking gas-making furnaces A and B, and operating the cracking gas-making furnaces by being connected in series; feeding the biomass and the urban waste as the raw materials from the tops of the furnaces in a continuous way or an intermittent way; and providing cracking heat energy through the reaction between the oxygen in a gasifying agent and residual carbon produced in cracking. According to the method disclosed by the invention, two cracking gas-making furnaces which are connected in series are adopted; the gasifying agent and the residual carbons produced by cracking the raw materials are reacted to generate heat for manufacturing the gas by cracking the biomass and the urban waste; and the method has the advantages that the gas can be manufactured by cracking the biomass and the urban waste at high temperature without external heat supply, and has capacity of preventing the raw materials from being moved out along with the gas.