A paperboard slitter machine includes a first housing, a rotary cutter movable in and out of the first housing for cutting a paperboard, a second housing, a passage defined between the first housing and the second housing for the feeding of the paperboard to be cut, a linking mechanism holding two roller brushes in the second housing and a driving device controllable to move the linking mechanism in carrying the roller brushes out of the second housing into the passage to press on the paperboard at two sides relative to the rotary cutter for enabling the paperboard to be accurately and smoothly cut by the rotary cutter.