Abstract: Fatigue damage assessment of tower-line systems is essential for ensuring power grid safety throughout their service life. This study investigates a typical three-tower two-line transmission system using an integrated analytical approach. A joint probability model of wind effects was established based on conditional wind speed distributions, developed from measured one-hour average wind speed and direction data. Fatigue damage was calculated for multiple wind directions, focusing on the stochastic behavior of component fatigue parameters and their effects on system reliability. Key findings reveal: comparative analysis with field measurements demonstrates that the Weibull distribution effectively characterizes conditional probability features of hourly mean wind speeds across different wind directions; correlation analysis reveals that stronger parameter correlations concentrate fatigue damage in tower legs; enhanced correlation among fatigue parameters significantly amplifies the random fluctuation range of tower fatigue life while elevating system-level uncertainty risks.