If a SQUID consisting of two junctions is placed in a cavity with sufficiently high Q-value, when the Josephson frequency of the junction ω=2eV0/h equals to one of the ei-gen frequencies ωr of the cavity, then the oscillating electromagnetic field in the form of standing wave excited by the junction current would react on the junctions and produce a series of steps of dc Josephson current within one flux quantum period Φ0. The theory also predicts that a series of new superconducting quantum interference phe-nomenons would be produced by Josephson currents of two junctions in the feed back field. The step height of the Josephson current will be inreased when both junctions are placed separately at the peak of the standing wave. The phase of the interference trace of SQUID will change by π/2 when one of the junctions is placed at the peak and the other is placed at the trough. The I-H trace of SQUID will degenerate to that of signle junction when one of the junctions is placed at the pack and the other is placed at the node. The step will not occur except that with n=0 yhen both junctions are placed separately at nodes.