Abstract: In the frame-forming process of advanced orbit system,the buffer capability and multiplexing efficiency directly affect the performance of the virtual channel access(VCA) sublayer and whole system.Different from the model of infinite buffer with long waiting-time,a statistics probability model of multiplexing protocol data unit(MPDU) is established based on Poisson distribution.A frame-forming model of equal-time based on the finite buffer is proposed,and the recursive formula about buffer capability,forming waiting-time and efficiency is given.Also the data from theoretical formula is compared with simulation data.The simulation results show that the recursive formula validate the real forming process very well.