The current design and optimization of the occupant restralnt system (ORS) are based on numerous actual tests and mathematic simulations. These two methods are overly time-consuming and complex for the concept design phase of the ORS, though they’re quite effective and accurate. Therefore, a fast and directive method of the design and optimization is needed in the concept design phase of the ORS. Since the alrbag system is a crucial part of the ORS, in this paper, a theoretical model for the vehicle alrbag is established in order to clarify the interaction between occupants and alrbags, and further a fast design and optimization method of alrbags in the concept design phase is made based on the proposed theoretical model. First, the theoretical expression of the simplified mechanical relationship between the alrbag’s design parameters and the occupant response is developed based on classical mechanics, then the momentum theorem and the ideal gas state equation are adopted to illustrate the relationship between alrbag’s design parameters and occupant response. By using MATLAB software, the iterative algorithm method and discrete variables are applied to the solution of the proposed theoretical model with a random input in a certaln scope. And validations by MADYMO software prove the validity and accuracy of this theoretical model in two principal design parameters, the inflated gas mass and vent diameter, within a regular range. This research contributes to a deeper comprehension of the relation between occupants and alrbags, further a fast design and optimization method for alrbags’ principal parameters in the concept design phase, and provides the range of the alrbag’s initial design parameters for the subsequent CAE simulations and actual tests.