Starting from the debugging problem of a multistage audio amplifier with triode，and aiming at the serious self-excited oscillation phenomenon of the output signal，the reason of parasitic feedback in the circuit is analyzed，and the internal resistance of the power supply is determined by the simulation software Multisim，and the solutions are given．The teaching case of feedback and self-excited oscillation is enriched，and an
effective method is provided for circuit analysis and troubleshooting．