Abstract: For deeper drilling depth and longer service life of MWD, a self-protection sealing device for the MWD instrument is designed given the defects of the combined static sealing structure. The relationship between rubber airbag expansion displacement and air pressure was studied through static expansion numerical simulation. On this basis, the reasons for different results due to different model sizes were analyzed, and the factors causing the different simulation results of the self-protection sealing device were discussed. The four independent variables of airbag diameter, width, wall thickness and inner cavity width were determined and simulated to explore their relationship with the rubber airbag expansion. Based on the static expansion simulation, a simple model of self-protection seal was established, and its sealing effect was simulated by fluid numerical simulation. The results show that the expansion displacement of the rubber airbag of the self-protection sealing device has a linear relationship with the pressure. The expansion displacement ratio k of the rubber airbag has a quadratic function relationship with the diameter and width of the airbag, and is inversely proportional to the inner cavity width and wall thickness of the airbag. The expansion displacement ratio k of the inner and outer rings of the rubber airbag is different, and with the increase of the independent variable parameters, the expansion displacement ratio k of the outer ring is gradually smaller than that of the inner ring. It can be deduced that there is a critical value of the structural size of the rubber airbag to reach the maximum expansion displacement ratio k . Under the protection of the self-protection sealing device, the fact that there is no water flow trajectory inside the simple self-protection sealing model proves that the self-protection sealing device is reliable. The critical value of the rubber airbag structure size provides a basis for designing the size of the self-protection sealing device to achieve the best sealing effect under the condition of the limited instrument space.