In order to solve the problems of complex structure and poor stability of transverse sealing device for ultrathin selfadhesive film, a set of pneumatic transverse sealing device based on the existing fresh food packaging machine was designed, which can complete the transverse sealing problem of ultrathin selfadhesive film. And the device completed the transverse sealing operation through the cooperation of six main mechanisms, including vertical conveyor, two up wind boxes arranged in parallel in front and back, cutting mechanism, unpowered idlers and lower folding mechanism which was composed of rollers, a cylinder and a lower wind box, which combined movement of pneumatic and mechanical structure to complete the transverse sealing operation. Then the mechanical model of packaging materials and transverse sealing device were established during transverse sealing process. After that, the structure of the lower wind box which was the key part of device was analyzed by Fluent software. The optimal parameters of the lower wind box were obtained as follows: the position of the air inlet and air outlet in the horizontal direction was symmetrically arranged, the height of the air cavity was 14mm, the size of outlet of the lower wind box was 3mm in diameter, and the number of outlets was 10. Furthermore, aiming to obtain the best parameters of the rear transverse sealing, the outlet pressure of the rear up wind box, the pressure of the horizontal air outlet of the lower wind box, the distance between the lower folding mechanism and the bottom of the packaging box were chosen as the experimental factors, and the transverse sealing rate was selected as the experimental index. Through the experimental research and the result analysis, the optimal aggregative index could be obtained under the condition that the outlet pressure of the rear up wind box was 113.3Pa, the pressure of the horizontal air outlet of the lower wind box was 85Pa, the distance between the lower folding mechanism and the bottom of the packaging box was -3mm. By verification of experiment, the maximum value of the transverse sealing rate was up to 99%, and the transverse sealing effect was flat and beautiful, which indicated that pneumatic transverse sealing device could satisfy operation demand of the fresh food packaging machine. 