The work aims to study the change characteristics of volume and pressure of airbag during deployment and optimize the design method and theory of airbag.Three methods were proposed to obtain the curve of airbag volume and pressure varying with the time during inflation and deployment from the perspective of theoretical analysis, experimental research and numerical simulation and the results acquired by the three methods were compared. The maximum error of balance pressure obtained from experiment and numerical simulation was less than 8%, and the relative error of airbag volume was 7%. Theoretical analysis and numerical simulation were consistent with experimental results, indicating the validity and correctness of the method. The inflation volume of airbag was divided into three processes:rapid deployment, slow volume growth and stable state. The pressure in the airbag was divided into initial stage, surge stage and equilibrium stage. The methods proposed are universal for general airbags and have certain engineering value.