Admission control plays an important role in OFDMA system. However, admission control becomes rather complex along with the increase of users' demand and the diversification of service types, so building an appropriate stochastic model is especially significant. The Petri net can distinctly describe the stochastic process. In this paper we build and analyze the Petri net model of admission control in OFDMA system. The simulation results demonstrate our theoretical analysis and indicate that the PN model is effectively to analyze the process of admission in OFDMA system.