In view of the instability phenomenon in a stable servo platform test, the instability failure mechanism was especially analyzed based on the working principle.According to the software interrupt settings of the servo controller,the functional analyses of each interrupt service program were performed in turn,and combined with on-site fault code expression,it was found that the interrupt function of UART0 was not turned off as required in the test state.And there was a problem that the priority of the interrupt was not reasonable,so the fault was located as a software reliability design defect.Combined with the existing equipment environment,the setting method of disabling UART0 interrupt service program and adding UART0 interrupt service program which could exit immediately once error triggered could effectively shield the external interference and improve the reliability of the equipment.By comparing with the two versions of the servo control software,the effectiveness of the measures was verified.By thinking about the risk caused by the software design defects in this extreme cases and its solution,some inspirations were given to the future software design work.