The transmission tower-line system, which is widely used in electric infrastructures across the world, generally possesses a small stiffness and low structural damping. The excessive vibration of a transmission tower-line system subjected to external excitations may induce a damage or failure, which has been frequently reported across the world. The effects of external loads on joint strength of transmission tower-line systems have not been examined in detail. In this regard, the force-bearing capacity of transmission tower joints is actively investigated in this study. A three dimensional analytical model of a tower joint is established with the aid of commercial package ABAQUS. The analytical model of bolt is developed by simulating the pre-stressed forces acting on structural members. A nonlinear contact force model for the interacting between bolts and member is presented to precisely simulate the contact effect subjected to wind loads. A real transmission tower-line system constructed in China is taken as an example to examine the feasibility and reliability of the proposed joint analytical approach. The parametric study is conducted to investigate the effects of various factors on joint strength, such as bolt diameter, bolt strength, pre-stressed force, friction coefficients and etc. The made observations demonstrate that bolt stress is linearly positively correlated with the bolt pre-tightening force under normal service load, while it is linearly negatively correlated with the bolt diameter. The bolt stress of skew member is linearly correlated with bolt pre-tightening force and bolt diameter under the critical load. When the external load increases from the normal service load to the critical load, the bolt stress of oblique members remains linearly related to the bolt pre-tightening force and the bolt diameter, while the bolt preload, bolt diameter, and friction factor have different effects on performance of bolt in major members. The friction interaction among contact surface has little effects on the strength of bolts.