In order to improve the testing accuracy, the intrinsic safety and the automatic level of the packing sealing inspection process step of certain military product, a locking device for the online packing sealing inspection has been designed and simulation calculation has been carried out. The results indicate that with an inspection pressure of 0.05 MPa, with copper and steel as the material for the inspection cavity and the locking device, the holding force of the locking device is higher than the force upward generated from the inspection cavity air pressure and the strength of the locking mechanism could meet the requirement. Moreover, the safety coefficient of the locking pin and locking seat of the locking device is respectively 12.3 and 7.17, both higher than the empirical safety coefficient value. It is concluded that the strength of the locking device could meet the design requirement and the locking device could work reliably and stably.