The invention discloses a sodium aluminate solution plug-flow decomposition reactor which belongs to chemical crystallization reactors. The invention aims to provide an oversaturated sodium aluminate solution inoculating crystal decomposition reactor, which has the advantage of low energy consumption and can enhance the crystal size of aluminium hydroxide. The sodium aluminate solution plug-flow decomposition reactor comprises a decomposing tank, a discharge chute, a discharge pipe, a rotating shaft and a lifting air pipe, wherein an electromotor and a reduction gear are fixed on the top of the decomposing tank; the discharge chute is fixed in the decomposing tank; the discharge pipe is vertically fixed on and communicated with the bottom of the discharge chute; the rotating shaft penetrates through the discharge pipe and is connected with the reduction gear; and the lifting air pipe is arranged in the discharge pipe. The invention is characterized in that a collection cylinder (6) fixed on the rotating shaft (3) is flexibly connected with the discharge pipe (4); at least one feed pipe (7) provided with a plurality of feed ports (8) is fixed on and communicated with the collectioncylinder (6); and a sealing element is arranged between the collection cylinder (6) and the discharge pipe (4). The invention has the advantages of large grain size of aluminium hydroxide, low equipment manufacturing cost and low power consumption, thereby being an ideal device for producing aluminium hydroxide.