The invention discloses a fluidized bed gasifier and a building method thereof and relates to the technical field of catalytic coal gasification and aims to lower construction difficulty and solve the problems that the refractory lining of an existing catalytic gasification fluidized bed gasifier is weak in alkaline corrosion resistance and friction resistance, and the refractory lining falls off easily. The fluidized bed gasifier comprises a gasifier body structure and a gasifier chamber, wherein the gasifier chamber comprises a gas chamber area, a dense-phase bed material area and a dilute-phase area, the gasifier body structure comprises a pressure-resistant shell and the refractory lining, and the refractory lining contains alkali-resistant materials and comprises a first lining part and a second lining part; the second lining part comprises a first refractory pouring layer contacting with the dense-phase bed material area and the dilute-phase area and a refractory brick masonry layer contacting with the inner wall of the pressure-resistant shell, and the first refractory pouring layer is located between the inner wall of the pressure-resistant shell and the refractory brick masonry layer. The fluidized bed gasifier has the advantages that the first lining part and the second lining part are manufactured in a sectioned manner, and the fluidized bed gasifier is applicable to the catalytic coal gasification technology.