The power-on self-test circuit fault of the discrete acquisition module is analyzed and located,which only leads to the error of the discrete input signal self-test,and does not affect the normal acquisition circuit func-tion.Through data acquisition and analysis,it locates that the fault is coused by interference triggering the latch-up of the switching device in the self-test circuit.The self-test circuit is improved,and the improved measures are ef-fective and can solve the fault problem through test verification.