Discussed the method of auto-relay adjustment of the PID regulator parameter. By analysing the limit periodic vibration resulting from the relay feedback,weestimate and adjust the parameter of the controlled target,and then accordingly makesthe adjustment of the PID parameter. The paper analyses the input and output waves ofthe controlled target under the influence of relay,and develops a formula for the calculation of the target parameter. The experimental findings have proved that the methodproposed here is convenient and reliable, and it is an effective method for automatic adjustment of PID parameter.