To research the effect of temperature and humidity on corona inception voltage gradient of UHV DC transmission line, a mini corona cage has been constructed and put in a manual environment room, which temperature and humidity can be controlled. At the same time, a corona inception voltage gradient measuring system by using vary methods such as current impule method, acoustic method have been set up too. The corona inception voltage gradient of a copper line have been measured in a mini corona cage not only at different absolute humidity from 3g/m 3 to 12g/m 3 but also at different temperature from 0°C to 35°C. The relationship between corona inception voltage gradient and temperature and humidity have been achieved.