High-accuracy evaluation of the effective spectral emissivity of blac kbody cavities is a hardness restricting the performance improvement of radiation temperature measurement and optical radiation measurement standards.In this paper,based on the thermal cavity reflection met hod,an experimental system for measuring the effective spectral emissivity of blackbody cavity is established. By comparing the reflectance of a standard sample and blackbody cavity from the spectral radiance of a dual-temperature aperture, the high precision reflectance of blackbody cavity is obtained.Ther efore,the cavity emissivity is obtained based on Kirchhoff′s law.The exprement of the normal effective emissivity of th e cavity is conducted,and the temperature changes from 30℃ to 80℃.The result shows that normal effective e missivity of the cavity is better than 0.9994,and the standard deviation is 0.016%.The consistency betwe en the experimental results and the Monte Carlo simulation results is better than 0.04%,which verifies the reliability of the measurement system.