The mooring system of a floating production storage offloading (FPSO) unit is crucial for its stable positioning at sea. Given that the mooring system must endure complex marine conditions over decades of service， component fatigue becomes a critical factor affecting its safety and reliability. This paper aims to study the fatigue life assessment methods for mooring systems， taking into account the characteristics of seasonal loads. Initially， based on actual monitoring data， we conducted research on fatigue life assessment methods for mooring systems. Through an in-depth analysis of long-term wind， wave， and current data in a specific area of the South China Sea， we discovered that the environmental loads in this region exhibit significant seasonal variation patterns， which are vital for accurately assessing the fatigue life of mooring system components. Therefore， this paper incorporates seasonal load characteristics into the fatigue life assessment model to more accurately simulate actual sea conditions and evaluate fatigue life. Using an FPSO in the South China Sea as a case study， the analysis results indicate that the fatigue life of the mooring chains from Line 1 to Line 4 is significantly lower than that of other mooring chain groups. This finding underscores the importance of considering load seasonal characteristics in mooring chain fatigue assessments， providing essential reference points for future engineering operations and maintenance.