The work aims to improve the efficiency and quality of automatic packaging production line, simplify the structure of transmission system and improve the performance of permanent magnet synchronous motor control system. A sensorless control technology based on sliding mode observer was proposed. The mathematical model of permanent magnet synchronous motor was established. On this basis, a sliding mode adaptive observer was designed to measure the stator current and rotor flux. At the same time, an I/F starting strategy was presented. The performance of the system was verified by simulation and experiment. The simulation and experimental results showed that the control method cannot only solve the motor starting problem, but also realize the speed tracking through the sliding mode observer, thus ensuring the accuracy of the rotor position. In practical application, the cutting point deviation of the transversal sealing and crosscutting mechanism can be controlled within 0.4 mm, and the average deviation was 0.2 mm. Speed sensorless control system has the characteristics of fast response speed, good speed regulation performance, strong robustness, etc., which can ensure the speed of the motor, position control precision and then reduce the redundancy and instability of the whole control system. It is suitable for packaging, simulation, printing and other occasions.