Yunnan-Guangdong ±800 kV DC transmission project is the first UHVDC transmission project in the world. The DC power transmission system must supply emergency power when the load increases or AC system malfunctions at the receiving end. It requires the DC power system to overload operating to increase its transmission capacity. This paper mainly studies the overload capability of the converter valve. It proposes a theory of calculating the overload capability based on the junction temperature and cooling capability of the valve cooling system. Moreover, with the specific data, it calculates and analyzes the overload capability of converter valve in Chuxiong-Suidong DC transmission project. The result indicates that this arithmetic can get the accurate overload capability of converter valve, which can provide reference and guidance to the design and operation of the practical HVDC transmission project.