By-products of SF6 react with water and lead to the formation of highly toxic decomposition products, which cause a reduction in breakdown performance. However, there is limited work carried out on the humidity effect on the DC breakdown characteristics of SF6 alternatives. This paper aims to characterize the effect of humidity of a pre-determined $C_3F_7$CN/CO2 gas mixture that matches the dielectric performance of SF6. Different humidity levels ranging from 109 to 8,306 ppmv were tested using negative DC for a 20‥ $C_3F_7$CN and 80‥ CO2 gas mixture at 2 bar (abs) and a hemisphere-plane configuration (hemisphere dia. of 8.8 mm) with gaps of 3 to 5 mm. Results indicate that breakdown voltage decreases with increasing humidity content within the $C_3F_7$CN gas mixture. X-ray photoelectron spectroscopy (XPS) analysis was conducted on the electrode surface after the experiment. The study showed that the deposited film on the electrode surface was mainly metal oxide and fluoride. As a result, the presence of humidity not only reduces the breakdown performance of the $C_3F_7$CN/CO2 gas mixture but also causes characteristic change to the surface of the electrode material.