Airbag system has so many advantages including small volume, superior cushioning performance and easy to control that it has been widely used in many fields such as heavy cargo airdrop, soft landing of spacecraft and so on. In this paper, an optimal design method of the airbag is proposed. First, based on the law of thermodynamics and the deformation assumption of airbag, a mathematical model of airbag landing process is established. The results of the model calculation of the cylindrical airbag is preferably consistent with the results of finite element analysis, which shows that the airbag mathematical model is reasonable and accurate. Second, on the basis of this model, the optimal design method of the airbag without rebound is proposed to solve the problem of rebound which will result in uncontrollable attitude and secondary shock in the landing processes. In this method, the evaluation index of airbag cushioning performance is determined, then the key design parameters which have significant impact on airbag cushioning performance are studied, and the optimization model of airbag under constraint with no rebound is subsequently established and solved. Third, by this method, a cylindrical airbag without rebound is obtained. Compared with the non-optimized one, the maximum impact acceleration of the optimized cylindrical airbag is smaller. Consequently, the effectiveness of the proposed optimal design method is verified.