Abstract: A gasifier including a vertically disposed furnace body, a feeder disposed in a middle part of the furnace body and communicating with the furnace body, one or two layers of microwave plasma generators, an external heater configured to supply external thermal energy for the gasifier, and a monitoring unit. The furnace body includes an upper nozzle for spraying vapor, a lower nozzle for spraying CO2/vapor, a syngas outlet disposed at a top of the furnace body. The upper nozzle for spraying vapor is disposed in a clearance zone of the furnace body, and the lower nozzle for spraying CO2/vapor is disposed in a bed zone of the furnace body. The monitoring unit is disposed close to the syngas outlet. The one or two layers of microwave plasma generators are disposed above the upper nozzle in the clearance zone of the gasifier.