In order to accurately measure the dielectric loss of electronic materials at a specific microwave frequency, a dielectric loss measurement method based on the defect model of microwave photonic crystal is proposed. Considering the microwave energy localization of the defective microwave photonic crystal, the dielectric under test is designed to be a defect layer within the microwave photonic crystal. The monotonic relationship between the transmission defect peak value and dielectric loss factor of the measured material is calculated by using transfer matrix method. When the dielectric loss factor varies from 0 to 1.0, the defect transmission peak value reduces from 0 to 0.21. As a result, when the defect transmission peak value is tested, and the dielectric loss factor can be obtained by using the calculated relationship between the transmission defect peak value and dielectric loss factor.