In order to extract the weak fault information of the pressure relief valve, the differential bispectrum operation was used to determine the fault state of the pressure relief valve by using the coupling property contained in the vibration signal. According to the AR bispectrum model, the complex thirdorder cumulants before and after differentiation were compared and analyzed. The frequency information added in the AR bispectrum after differentiation had a certain influence on the result of fault diagnosis. After several differentials, it is found that the weak coupling signal components in the signal can be extracted by increasing the number of differentials,so more fault information can be extracted. The experimental results show that by changing the number of differential, the vibration signals whose fault information is not obvious can be distinguished, thus the accuracy of fault diagnosis is improved.