The invention relates to a prepressing device for detecting the density of a strips encasement. The prepressing device is composed of a rack (1), a hydraulic oil cylinder (2) arranged on the rack (1) and a pressure head (3) connected with a piston end of the hydraulic oil cylinder (2), wherein the pressure head (3) comprises a pressure head frame (4), a connecting piece (5) which is fixed on the top end of the pressure head frame (4) and is connected with the piston end of the hydraulic oil cylinder, a pressure sensor (7) fixed on the bottom surface of the pressure head frame (4) as well as a probe (6) connected with the rear end of the pressure sensor and a control system (10) connected with the pressure sensor (7). The prepressing device provided by the invention has the following beneficial effects: (1) the pressure sensor is utilized to detect the density variation of a finished strips encasement, thereby avoiding the damage to a smoke box and tobacco through a nine-point sampling method; and (2) the encasement density in a prepressing process is detected in real time, a system can find a problem in real time and can correct in time, and the real-time control of the encasement density variation becomes possible.