In the process of porcelain insulator infrared detection, the missing judgment may happen due to the temperature difference of a zero value insulator failing to reach the threshold specified by the relevant standards. In order to improve the accuracy of the infrared detection, this article proposes a new method combining the iron cap temperature difference threshold method and the temperature gradient correlation coefficient method. Infrared detection field experiment of disc suspension insulators in substation is conducted.The test results show that under the suitable environment conditions, the zero value insulator can be confirmed by studying the dispersion characteristics of temperature gradient distribution correlation coefficients, while the result of difference threshold method for iron cap temperature is unstable. Thus, the missing judgment probability can be decreased. By contrast, under the unsuitable environment conditions, the dispersion of the temperature gradient correlation coefficient increases with the increase of the ambiguity for the temperature difference threshold method. Reduction occurs on the discrimination degree of the temperature gradient correlation coefficient between the normal insulator string and the zero value insulator string.