Facing the goal of "double carbon", the construction of a novel power system is the only way for the transformation and upgrading of power grid. With the construction and development of novel power system, the power grid has put forward higher requirements for toughness and transmission line operation reliability. In particular, extreme weather can easily cause power outages and bring huge losses to the national economy and lives of people. The overload of overhead transmission lines caused by extreme weather is the main cause of power outage. Based on load-strength characteristics of transmission lines and probability and statistical theory, a risk assessment method for the operation of overhead transmission lines under extreme weather is proposed. The time-varying combined load model is constructed with extreme weather and meteorological data, structural parameters of power network and geographic information data. In this paper, a time-varying risk propagation model considering time-varying combined load is established, fault set is obtained by fault simulation using Latin hypercube sampling method, a transmission line risk assessment index is proposed to characterize the real-time risk and time-varying risk of transmission lines to guide the operation and maintenance of transmission lines. Take an actual transmission line of Zhejiang Power Grid as an example, the results show that the proposed extreme weather transmission line risk assessment method can effectively evaluate the risk situation of transmission line operation. The reconstruction scheme of line operation and maintenance can effectively improve the toughness of power grid.