The gene gun is an important apparatus for continuous gene transfection and is widely regarded as a common delivery device for gene therapy. The gene transfer process is a highly dynamics process. To achieve effective treatment, the gene transfer process needs to be well understood and controlled. Unfortunately, no dynamics model is available to analyze the process in the literature. In this paper, a dynamics model of the gene gun is proposed. Based on the mathematical model, simulation studies have been conducted. Relations of penetration depth with helium pressure, acceleration distance, macro-carrier mass, and micro-carrier radius are discussed. The results agree well with experimental results in the literature