A sheet medium flattening device includes a mounting frame configured to mount and support following components; an annular passage, including an annular inner passage plate and an annular outer passage plate, a heating module arranged in a hollow inner cavity of the inner passage plate to supply heat for the inner passage plate; at least one pinch roller arranged at an outer side of the annular guiding plate and realizing clamping with the inner passage plate through a hole slot arranged in the annular guiding plate; a reversing mechanism configured to selectively convey banknotes between the conveying passage and the annular passage of the flattening device; a driving device configured to drive the inner passage plate to rotate; and a control system configured to control a temperature of the heating module and control a selectively switchover of the reversing mechanism.