The invention discloses a process and an apparatus thereof for low-temperature cracking and high-temperature gasification of biomass. By adopting superheated vapor steam as oxidizing agent and an energy carrier, the process comprises the following steps of: firstly, carrying out the low-temperature cracking on the biomass fuel in the temperature region of 500-800 DEG C and obtaining the crude synthetic gas and the coke without containing alkali metal oxides; secondly, carrying out the high-temperature gasification on the crude synthetic gas and the coke in the temperature region of 1200-1600 DEG C to obtain the initial synthetic gas without containing tar component; finally, cooling, dust-removing, deacidifying and drying the generated initial synthetic gas, i.e., acquiring the pure synthetic gas of high quality. The apparatus mainly consists of a cracking furnace, a gasifying furnace, a low-temperature plasma torch heater, a high-temperature plasma torch heater, a water storage tank,a water delivery pump and a heat exchanger and the like. The process of the invention has the advantages of easy control, low energy consumption and investment cost and high efficiency of cooled coalgas; the generated synthetic gas has high calorific value and does not contain tar and alkali metal compounds. The invention is suitable for not only biomass gasification combined cyclic power generation, but also preparation of biomass liquid fuel.