Disclosed are a sheet storage device and a controlling method thereof the sheet storage device including a storage unit, an adjustment assembly and a control portion. The storage unit comprises a mounting frame, a plurality of support brackets configured to place a plurality of sheets and two shiftable mechanisms mounted on the mounting frame and spaced apart in an upper and lower direction. The support brackets are mounted between the two shiftable mechanisms and spaced apart in a position shifting direction of the two shiftable mechanisms, and two adjacent support brackets thereof are constructed to define a storage space for placing the sheet. Each support bracket is configured such that upper and lower ends of the support bracket are adapted to be shiftably engaged with the two shiftable mechanisms respectively, so as to adjust the distance between the two adjacent support brackets. The adjustment assembly is engaged with at least one of two adjacent support brackets to adjust the size of the storage space. The control portion is connected to the adjustment assembly to control the adjustment assembly according to received user instructions.