Abstract: The superconducting(SC) accelerator has high quality factor Q, and the decay time of the field excited by beam is much longer than that in normal conducting cavity. Thus, beam induced field has far consequences for power dissipation and beam stability. In the paper, this interaction process is analyzed considering a point charge moving on axis through a cavity. Based on it, the beam loading in PKU SCAF has been researched. With the 10 kW input power, the optimal average current in the 9 cell SC cavity of PKU SCAF is 1.5 mA, accelerating voltage is 6.6 MV, and β factor is 4.5×10 3.