The complexity and remarkable three-dimensional properties of mountainous terrain wind field are the key problems of wind-resistance design of transmission lines in mountainous areas. The properties of three-dimensional mean wind field and fluctuating wind field of typical single mountain terrain and parallel-sided valley terrain are studied by CFD numerical simulation and the wind tunnel test. Three-dimensional mean wind field at each point along the transmission lines was given by CFD numerical simulation because of the limited measuring points and wind speed in wind tunnel test. Based on the turbulence intensity, turbulence integral scale, attenuation coefficient along the lines and related fluctuating wind field factors obtained from wind tunnel test, three-dimensional fluctuating wind speed time-histories at each point along the transmission lines were generated by POD method, and the wind field properties along transmission lines cross the ridge and valley were analyzed and compared. A 3-span finite element model was built to study the influence of the mountainous terrain wind field properties on the wind-induced swing response of conductor based on the time domain method. The results show that wind speed at the top of the mountain and at the valley area increase significantly, and the climbing airflow generates updraft in the local position. The speed-up effect results in a significant increase in wind-induced swing of all lines along the ridge of single hill as well as the climbing sections of spanning valley transmission lines. The updraft mainly increases the vertical displacement of the conductors. Dynamic response factor of wind-induced swing angle of insulator string should be taken between 1.2 and 1.3 when considering the effects of fluctuating wind in mountainous terrain. This research results can provide reference for the wind-induced swing resistant design of transmission lines in mountainous areas.