The sealing effect of packing is one of the main factors that affect the product packing quality. In order to realize the automatic packing of certain product, an online packing sealing inspection device has been designed and fabricated with steel and copper as the material for the detecting cabin and locking mechanism. Conceptual calculation and simulation calculation have been carried out for the locking mechanism of the online inspection device. With an air pressure of 0.05MPa for the detection of the online inspection device, the holding force of the designed locking mechanism is higher than the force upward generated from the detection air pressure, which together with the strength of the locking mechanism could meet the requirement. Through the simulation calculation, it has been confirmed that the strength of the locking mechanism could meet the design requirement and work reliably and stably.