A gasifier, including a vertically disposed furnace body, a monitoring unit, and a microwave plasma generating device. The furnace body includes a material and fuel inlet, a syngas outlet, an oxygen/vapor inlet, and a slag outlet. The furnace body has a clearance zone in an upper part thereof and a fixed bed zone in a lower part thereof. The slag outlet is disposed at the bottom of the furnace body. The monitoring unit is disposed close to the syngas outlet. At least one microwave plasma generating device is disposed on the furnace body.